lib: Add msghdr.[ch]

lib: Add msghdr.[ch]

This is a little set of routines to deal with the ugly fd-passing macros.

This patch is the first step assisting the creation of msghrds for sending fds.
Receiving fd helpers will follow later.

The basic idea behind these routines is that they fill a variable-sized buffer.
They are supposed to be called twice per msghdr preparation. First with a
0-sized NULL output buffer to calculate the required bufsize, and then a second
time filling in the buffer as such.

This does not take care of the old msg_accrights way of passing file
descriptors. CMSG/SCM_RIGHTS is standardized for quite a while now, and I
believe this intreface can be made to also take care of msg_accrights if
needed.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>