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

This call was broken since commit
9a5bcc6d because the
`UniqueSocketDescriptor` had already been moved.

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

Closes #169

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

bind fails with an "address already in use" error if the socket path
already exists.

This was broken by cbb595ba.

Manages memory automatically.

The implicit cast operator can be very dangerous.

Must return -1 on error, not 0.  0 is root.

Make sure glibc gives us all features.

Users now have to call Close() explicitly.  This simplifies using the
class, as most users have automatic socket management already, and
Steal() had to be used often.

No GLib memory allocation.