scubainit: Make SCUBAINIT_(UID|GID|UMASK) optional
scubainit: Make SCUBAINIT_(UID|GID|UMASK) optional

- If either of UID or GID are set, they both must be set, else error
- If both UID and GID are set, change_user() has an effect
- If UMASK is set, the umask will be set

JonathonReinhart/scuba,JonathonReinhart/scuba,JonathonReinhart/scuba