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