Implement decoding of pidfd_open syscall

Implement decoding of pidfd_open syscall

... introduced by Linux kernel commits v5.3-rc1~142^2~2
and v5.3-rc1~142^2~1.

* pidfd_open.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* pathtrace.c (pathtrace_match_set): Add SEN_pidfd_open.
* linux/syscallent-common.h [BASE_NR + 434]: Wire up pidfd_open.
* NEWS: Mention this change.
* tests/pidfd_open.c: New file.
* tests/pidfd_open-P.c: Likewise.
* tests/pidfd_open-y.c: Likewise.
* tests/gen_tests.in (pidfd_open, pidfd_open-P, pidfd_open-y): New
entries.
* tests/pure_executables.list: Add pidfd_open, pidfd_open-P, and
pidfd_open-y.
* tests/.gitignore: Likewise.