pam_namespace: add mntopts flag for tmpfs mount options

modules/pam_namespace/pam_namespace.h: Add mount_opts member to polydir
structure.
modules/pam_namespace/pam_namespace.c (del_polydir): Free the mount_opts.
(parse_method): Parse the mntopts flag.
(ns_setup): Pass the mount_opts to mount().
modules/pam_namespace/namespace.conf.5.xml: Document the mntopts flag.