mount: parse default values correctly

Commit 88c22f92 converted the configfile.c routines
to use the parse_opt interfaces which broke how
default values from nfsmount.conf are managed.

Default values can not be added to the mount string
handed to the kernel. They must be interpreted into
the correct mount options then passed to the kernel.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1912877

Signed-off-by: Steve Dickson <steved@redhat.com>