Fix fsearch by using a sharded file buffer

See merge request !352

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 i4/manycore/emper!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 i4/manycore/emper!345

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

See merge request !343

Make subprojects

See merge request i4/manycore/emper!344

io: make max unbounded io worker configurable

See merge request i4/manycore/emper!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

This required to break an include cycle between Fibril and
LockedQueue.

Improve LinuxVersion

See merge request i4/manycore/emper!342

Memory manager

See merge request i4/manycore/emper!339

A Linux version including non-digits, like 5.15.23-gentoo-dist, is
very common and nothing we should warn about.

[Runtime] Prefix EMPER workers names with EMPER

See merge request i4/manycore/emper!341

Also add error handling to uname() call.

Add stack-usage stats

See merge request i4/manycore/emper!338

add Context out of bounds write test

See merge request i4/manycore/emper!336

EMPER's Runtime now installs the SIGSEGV handler that checks if the
segmentation fault was caused by hitting the guard page. If the guard
page was hit, then the handler prints an error message.

Also make ContextOutOfBoundsTest more robust by not creating an
infinite recursion, which may be optmized out by the compiler.

[Emper.hpp] Remove unnecessary 'static' keyword

See merge request i4/manycore/emper!337

Add an SIGSEGV handler to the ContextOutOfBoundsTest to ensure that
the received SIGSEGV is caused by accessing the guard page.