Decode file descriptors passed via SCM_RIGHTS control messages

* net.c (printcmsghdr): Print descriptors from SCM_RIGHTS control
messages using printfd.
* tests/scm_rights.c: New file.
* tests/scm_rights-fd.test: New test.
* tests/Makefile.am (check_PROGRAMS): Add scm_rights.
(TESTS): Add scm_rights-fd.test.
* tests/.gitignore: Add scm_rights and uio.
5 files changed