fix lockless io-stealing

See merge request !357

The uninitialized continuation pointer may be returned without being
written resulting in dispatching a not valid fiber pointer.

If a CQE is stolen but no fiber is blocked on the corresponding future
IoContext::getContinuationsFromCompletions will call future->completeAndGetContinuation
which will set the future's completion but will not return a continuation.

If no continuation is returned from Future::completeAndGetContinuation
the continuation buffer is not advanced by IoContext::getContinuationsFromCompletions
(emper/io/IoContext.hpp:259).

IoContext::reapSingleCompletion does not check if a continuation was
created and assumed that continuation contains a valid Fiber* if
a CQE was successfully stolen.

Initialize continuation with nullptr, which is a valid Fiber*.

Minor changes of top-level files

See merge request !356

Extend emper::printInfo()

See merge request !355

Add more CL Queue implementations

See merge request i4/manycore/emper!354

Work-Stealing queue fill-size stats

See merge request i4/manycore/emper!353

The calculation was previously done using integers, this lead to
MEMORY_MANAGER_VICTIM_PERCENTAGE to become 0.

The MemoryManager uses lib::adt::wsqueue::Default as work-stealing
queue implementation. The reference to WsClQueue would break if e.g.,
ws_queue_default=cl4, since the type would not exist.

In case pushing to the local work-stealing queue fails because the
queue is full, and that failure is not fatal as it is in Fibril, emit
a warning once.

This result also contains the observed queue size at the moment of the
push.

Fix fsearch by using a sharded file buffer

See merge request i4/manycore/emper!352

Also rename 'res' to 'pushed' and only notify about new work if there
was work pushed.

Each worker thread has it own stringstream per ShardedFileBuffer.
All streams are joined and flushed to the file when a ShardedFileBuffer
is destructed.

Flushing a ShardedFileBuffer is not thread-safe and must not be called
concurrently to any modifications to the ShardedFileBuffer.

Use a ShardedFileBuffer to accumulate and print all paths during fsearch.

[StealingResult] Improve docstring

See merge request !349

fsearch: write only found files to stdout and use implicit file offset

See merge request i4/manycore/emper!350

Use std::function based factory

See merge request i4/manycore/emper!71

In the emper-fs-eval I would like to validate the output of the fsearch
variants against a list of known files. Not writing any log messages to
stdout make that easier.

Use -1 as the offset of the write to stdout.
Passing offset -1 means to use the file's implicit offset just like write(2).
When using a tty as stdout offset 0 is fine but when redirecting stdout
to a file 0 will cause that previously found paths will always be
overwritten.

[LinuxVersion] Use "Construct on first use" idiom and add mutex

See merge request i4/manycore/emper!348

Use the "Construct on first use" idom for UTS Release (aka Linux
version) initialization. The previous implementation was fragle and
could lead to segfaults like

    __s2=0x7fffffffe480 "5.16.10-arch1-1", __s1=<optimized out>)
    at /usr/include/c++/11.2.0/bits/char_traits.h:409
    __n=<optimized out>, __s=0x7fffffffe480 "5.16.10-arch1-1", __d=<optimized out>)
    at /usr/include/c++/11.2.0/bits/basic_string.h:359
    __n=<optimized out>, __s=0x7fffffffe480 "5.16.10-arch1-1", __d=<optimized out>)
    at /usr/include/c++/11.2.0/bits/basic_string.h:354
    __str=..., this=0x7ffff7fbd2a0 <emper::lib::LinuxVersion::globalVersion[abi:cxx11]>)
    at /usr/include/c++/11.2.0/bits/basic_string.h:739
    at ../emper/lib/LinuxVersion.cpp:46
    at /usr/include/c++/11.2.0/ext/new_allocator.h:79

if globalVersion's constructor was not yet called. This is the so
called "static initialization order fiasco" in C++.

While the mutex may not be strictly requires, as we do not call
LinuxVersion's non-argument constructor nor getUtsRelease()
concurrently, it can not hurt to have one.

do not define futex_waitv if already done in linux/futex.h

See merge request !345

Make the implementations of the work-stealing queue(s) selectable

See merge request i4/manycore/emper!343

Make subprojects

See merge request !344

io: make max unbounded io worker configurable

See merge request !340

Since Linux 5.15 io_uring can limit the number of iow threads created
using IORING_REGISTER_IOWQ_MAX_WORKERS.

Bump liburing wrap to version 2.1 to use
io_uring_register_iowq_max_workers.

Expose this via the meson variable io_unbounded_iow_max and the
environment variable EMPER_IO_UNBOUNDED_IOW_MAX.

See for an detailed explanation:
https://blog.cloudflare.com/missing-manuals-io_uring-worker-pool