Implement decoding of move_mount syscall

Implement decoding of move_mount syscall

... introduced by Linux kernel commits v5.2-rc1~141^2~8,
v5.2-rc1~20^2~1, and v5.2-rc1~20^2.

* move_mount.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* pathtrace.c (pathtrace_match_set): Add SEN_move_mount.
* xlat/move_mount_flags.in: New file.
* linux/syscallent-common.h [BASE_NR + 429]: Wire up move_mount.
* NEWS: Mention this change.
* tests/move_mount.c: New file.
* tests/move_mount-P.c: Likewise.
* tests/gen_tests.in (move_mount, move_mount-P): New entries.
* tests/pure_executables.list: Add move_mount and move_mount-P.
* tests/.gitignore: Likewise.