Always fall back to socket()/bind() when os.mknod(...S_IFSOCK) fails.

Always fall back to socket()/bind() when os.mknod(...S_IFSOCK) fails.

Previously bup would use socket()/bind() instead of os.mknod(... |
stat.S_IFSOCK) on Cygwin, but this issue isn't Cygwin specific.
Remove the platform conditionalization, and fall back to
socket()/bind() any time mknod() fails with EINVAL.

Thanks to Robert Edmonds <edmonds@debian.org> for reporting the
relevant failure on a Debian kFreeBSD buildd.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>