seccomp: fix decoding of sock_fprog and sock_filter structures

seccomp: fix decoding of sock_fprog and sock_filter structures

Always print struct sock_fprog.len.
Fix printing of unfetchable elements in sock_filter array.
Fix printing of large sock_filter arrays.

* seccomp.c (decode_fprog): Rewrite into decode_seccomp_fprog and
print_seccomp_fprog.
(print_seccomp_filter): Replace decode_fprog with print_seccomp_fprog.
* tests/prctl-seccomp-filter-v.c (main): Update.
* tests/seccomp-filter-v.c: New file.
* tests/seccomp-filter-v.test: New test.
* tests/.gitignore: Add seccomp-filter-v.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add seccomp-filter-v.test.