Implement decoding of NS_* ioctl commands

Implement decoding of NS_* ioctl commands

* configure.ac (AC_CHECK_HEADERS): Add linux/nsfs.h.
* defs.h (DECL_IOCTL(nsfs)): New prototype.
(setns_types): Make global.
* ioctl.c (ioctl_decode): Call nsfs_ioctl for 0xb7 code.
* nsfs.c: New file.
* nsfs.h: Likewise.
* Makefile.am (strace_SOURCES): Add them.
* tests/ioctl_nsfs.c: New file.
* tests/ioctl_nsfs.test: Likewise.
* tests/.gitignore: Add ioctl_nsfs.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add ioctl_nsfs.test.
* NEWS: Mention this change.