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!

The new implementation is copied from another project and is
BSD-licensed.  It is exception-safe and can parse IPv6 scope ids with
interface names.

first call fchmod() to prevent TOCTTOU, then apply permissions using
chmod()

Fixes the NFS client build, closes #305.

This fixes a build issue on Haiku as it does have sin_len.
Tested on Linux as well.

For some reason AC_CHECK_MEMBER doesn't generate the proper define
in config.h.in, so I used AC_CHECK_MEMBERS.

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

Closes #169