Define static helper functions serialize/deserialize in
fhandler_socket_unix.cc. These will be used to support sending file
descriptors via SCM_RIGHTS control messages.
The serialize function creates an 'fh_ser' structure that contains a
copy of the fhandler associated with the file descriptor, with all
allocated memory freed. The structure also contains the Windows pid
of the current process, which deserialize can use for duplicating
handles.
The deserialize function reconstructs an fhandler from an fh_ser
structure, with the handles duplicated into its own process.
For now, serialization and deserialization are fully implemented only
for disk files, and even in that case there are many FIXMEs that need
attention.
