net: enhance decoding of getsockopt(SO_PEERCRED)

net: enhance decoding of getsockopt(SO_PEERCRED)

* net.c (print_ucred): Rename to print_get_ucred, change decoder
to match the kernel behaviour: getsockopt syscall accepts any
non-negative *optlen and writes MIN(sizeof(struct ucred), *optlen)
bytes of data.
(print_getsockopt): Replace print_ucred with print_get_ucred.
* tests/so_percred.c: Include <string.h>.
(main): Update expected output.