util_runcmd: Free the fde in event handler.

util_runcmd: Free the fde in event handler.

Free the fde in the event handler to prevent the event triggering again
While not strictly necessary in this case, this code serves as an
example of the usage of tfork.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13037

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Sat Sep 16 23:50:27 CEST 2017 on sn-devel-144