ports-mgmt/portmaster: allow unprivileged users to fetch distfiles

Allow portmaster -F to be run without requesting root privileges.

Keep the testing of the user id as a separate test in order to not
execute $PM_SU_CMD if $NO_ACTION" or $FETCH_ONLY is set.

The distfiles directory must be writeable to take advantage of this
change.

PR: 226506
Reported by: David Wolfskill