Implement decoding of fsmount syscall

Implement decoding of fsmount syscall

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

* fsmount.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* pathtrace.c (pathtrace_match_set): Add SEN_fsmount.
* xlat/fsmount_cmds.in: New file.
* xlat/mount_attr_atime.in: Likewise.
* xlat/mount_attr_flags.in: Likewise.
* linux/syscallent-common.h [BASE_NR + 432]: Wire up fsmount.
* NEWS: Mention this change.
* tests/fsmount.c: New file.
* tests/gen_tests.in (fsmount): New entry.
* tests/pure_executables.list: Add fsmount.
* tests/.gitignore: Likewise.