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

Improve LinuxVersion

See merge request !342

Memory manager

See merge request !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 !341

Also add error handling to uname() call.

Add stack-usage stats

See merge request !338

add Context out of bounds write test

See merge request !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 !337

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

Add emper::printInfo()

See merge request !333

The test is a simple infinite recursion hitting the guard page.
The new guard-page-test-runner skips the test if emper::STACK_GUARD_PAGE
is false.

[Context] Add option for guard page at the bottom of stack

See merge request !335

[meson] fix context_alignas option

See merge request !334

The used variables are named assume_*.
And meson does not support concatenating different variable types using
'+' use string.format instead.

[Context] Make context size configurable

See merge request !331

Extend README and switch to one line per sentence

See merge request !329

Also keep the context size at 64 KiB (there was a comment that
errorneously indicated that the context size is 4 MiB).

do not warn about missing atomic::{wait,notify}

See merge request !332

This allows emper to be build with --meson-fatal-warnings on in our i4lab.

I do not see a reason to do so any more (there probably never was).

Use constexpr for memory manager in context manager

See merge request !330