xfstests: add nsexec user namespace helper

Add new program nsexec to facilitate creating/entering a user namespace. The
original source for the program is https://lwn.net/Articles/539940. I added
the -s option to become "root" in the user namespace.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Reviewed-by: <rjohnston@sgi.com>
Signed-off-by: <rjohnston@sgi.com>

.gitignore

@@ -64,6 +64,7 @@
 /src/mmapcat
 /src/multi_open_unlink
 /src/nametest
+/src/nsexec
 /src/permname
 /src/preallo_rw_pattern_reader
 /src/preallo_rw_pattern_writer