This is the final piece of the series to establish io_uring support on
Linux.

MPD doesn't need io_uring for its efficient bulk I/O support, but to
allow file I/O to be cancelled.  This is a big problem on CIFS/NFS
mounts where processes sleep uninterruptable if the file server
disappears, deadlocking MPD.

With io_uring, a flaky NFS connection allows MPD to continue to work
(even though there are still deadlocks inside MPD which need to be
addressed).

This plugin does not yet use cancellable `open()` using
`IORING_OP_OPENAT`.  This will be implemented later.

Lots of other optimization opportunities for io_uring are still
missing as well - for example the database update could benefit a lot,
but unfortunately, io_uring doesn't have `readdir()` support just yet.

This makes ccache more efficient when recompiling with different
plugins.

Move code from ParseCommandLine().

_exit and std::_Exit are identical, expect the latter is standard C++.

Added several functions to the std namespace as a result of headers.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with llvm-namespace-comment
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-avoid-const-params-in-decls
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with modernize-redundant-void-arg
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-simplify-boolean-expr
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Split several printf() calls to make it easier to deal with all those
#ifdefs.

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

Modeled after std::filesystem::operator/() from C++17.

To get udisks2 support started, this commit contains the configure.ac
option and a "neighbor" plugin which shows block devices.  Later, this
will allow mounting removable media with a new storage plugin.

From https://github.com/CM4all/libcommon/commit/a7cef404c1f7eedff1a629da48f77491218ee281

To be used by upcoming code.

These have no use, exception for creating an artificial dependency
from input_plugins[] to all archive plugins.

Prepare for option values.

Allow the caller to use a simple "for" loop without checking
arguments.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169