Implement decoding of fsopen syscall

Implement decoding of fsopen syscall

... introduced by Linux kernel commits v5.2-rc1~141^2~5,
v5.2-rc1~20^2~1, and v5.2-rc1~20^2.

* fsopen.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* pathtrace.c (pathtrace_match_set): Add SEN_fsopen.
* xlat/fsopen_flags.in: New file.
* linux/syscallent-common.h [BASE_NR + 430]: Wire up fsopen.
* NEWS: Mention this change.
* tests/fsopen.c: New file.
* tests/gen_tests.in (fsopen): New entry.
* tests/pure_executables.list: Add fsopen.
* tests/.gitignore: Likewise.