Haiku has struct ucred but no SO_PASSCRED (yet).

Replaces GLib's GError.

Use WinSock errors on Windows.

Ensure that WINVER is defined early enough, so other system headers
won't fall back to their default value.  Specifically, this solves a
build failure (-Werror) with mingw-w64 ("WINVER redefined").

In a C file, that is too late, because _mingw.h defaults to 0x0502,
and ours would be illegal redefintion.

On Win32, the third setsockopt parameter has type (char *) while on POSIX
systems it is (void *). However, given that it is a no-op cast to go from a
char pointer to a void pointer, we can cast to a char pointer (with a
possible const modifier) on all platforms and satisfy the compiler.
Signed-off-by: Dan McGee <dan@archlinux.org>

Wrap close(), use closesocket() on WIN32/WinSock.

If a connected host disappears without our knowledge, as can happen over
wireless or a hibernating machine, we continue to hold the port open waiting
for messages. Because we never try to send anything down this now-broken
pipe, the connection will sit idle taking up a slot in our allowed incoming
connections list.

If enough of these happen, an unintended Denial of Service takes place,
where all connection slots are filled with now-broken, never ending
connections. Setting the TCP keepalive option at least allows these to time
out after the default two hours, which is sufficient in the non-malicious
case.
Signed-off-by: Dan McGee <dan@archlinux.org>

In the winsock headers, the setsockopt() argument is declared as
"const char *", not "const void *".

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

Changed the wrappers for pipe(), socket(), accept().  On WIN32, this
does not work for pipe().

Added the "fd_util" library, which attempts to use the new thread-safe
Linux system calls pipe2(), accept4() and the options O_CLOEXEC,
SOCK_CLOEXEC.  Without these, it falls back to FD_CLOEXEC, which is
not thread safe.

This is particularly important for the "pipe" output plugin (and
others, such as JACK/PulseAudio), because we were heavily leaking file
descriptors to child processes.

g_set_error() is a printf-like function, and expects a format string.
Using the return value of gai_strerror() is unsafe.

g_strerror() is more portable, and guarantees that the returned string
is UTF-8 encoded.

Moved code from listen_add_address() (listen.c) to socket_util.c.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

On some systems, the macro IN6_IS_ADDR_V4MAPPED() is not available.
Don't try to convert IPv6 to their IPV4 equivalents in this case.

Windows doesn't have the standard headers sys/socket.h and netdb.h.