autofs-5.0.7 - fix add null check in parse_server_string()

In a recent patch that added an error check for the case there's no ':' server
name delimiter parse_server_string() braces were not added which lead to an
incorrect parse error.
autofs-5.0.7 - check for protocol option

When a specific protocol is requested in the mount options only
that protocol should be probed for.
autofs-5.0.7 - use ulimit max open files if greater than internal maximum

When setting the maximum number of allowed file handles the current setting
should be checked before setting it. If the ulimit command has been used to
increase the maximum to larger than what automount would ask for then honour
it.