Implement decoding of linux socket filter programs

Implement decoding of linux socket filter programs

* bpf_sock_filter.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* xlat/skf_ad.in: New file.
* defs.h (decode_sock_fprog, print_sock_fprog): New prototypes.
* fetch_bpf_fprog.c (get_bpf_fprog_size): New mpers printer.
* net.c (print_getsockopt): Use decode_sock_fprog to print
socket filter programs for SO_GET_FILTER socket option.
(print_setsockopt): Use decode_sock_fprog and get_bpf_fprog_size
to print socket filter programs for SO_ATTACH_FILTER
and SO_ATTACH_REUSEPORT_CBPF socket options.
* NEWS: Mention this.