osutil: disable compilation of recvfds() on unsupported platforms
It appears that Solaris doesn't provide CMSG_LEN(), msg_control, etc. As
recvfds() is only necessary for chg, this patch just drops it if CMSG_LEN
isn't defined, which is the same workaround as Python 3.x.
https://hg.python.org/cpython/rev/c64216addd7f#l7.33