This commit fixes a build issue on systems with a prototype for setns
in their header files but no working setns is detected by configure.
gdb/ChangeLog:
PR gdb/18957
* nat/linux-namespaces.c (setns): Rename from this ...
(do_setns): ... to this. Support calling setns if it exists.
(mnsh_handle_setns): Call do_setns.