_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>

The former was deprecated with C++14. The standard says they are the same:

The contents of the header<csignal>are the same as the C standard library
header<signal.h>.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

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

Don't use the bad `FatalError` library.

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

Closes #169

The initgroups() manpage says we need to check for _BSD_SOURCE.  The
thing is that glibc deprecated this macro, and doesn't define it
anymore, effectively breaking all MPD supplementary groups.

The real fix is to check for initgroups() availability at configure
time, instead of relying on the deprecated _BSD_SOURCE macro.

Haiku does not dump core, it just starts the debugger.

Keep the parent process around until MPD has finished initializing.

This is important for libraries that are allergic to fork(), such as
libupnp.

Prepare for more advanced daemonization code, which will not work with
daemon().  Let's just require fork().  Everybody who supports daemon()
also supports fork().

We can assume that initgroups() would be a no-op in that case, however
initgroups() is not allowed for unprivileged users anyway.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Migrate all callers to use Path directly, instead of doing the
conversion in each caller.