Add support to decode sockets descriptor 'paths' for network calls.

* pathtrace.c: Add decoding for network-related file descriptors to
pathtrace_match with an fdmatch call for allTRACE_NETWORK calls that
have sockfd as arg[0]. No decoding for socket and socketpair calls that
take no descriptors.
* net.c: Add printing of decoded socket descriptors for syscalls that
have such desc as arg[0] instead of printing a bare int.
* tests/net-fd: New test file to test proper decoding of sockfd's.
* tests/Makefile.am (TESTS): Add net-fd to test suite.