feature request for portmaster: check for permissions on --check-depends and friends

[Eitan Adler]

When I run portmaster with --check-depends or --check-port-dbdir  as a
non-root user portmaster will continue to attempt to write to the db
dir.
For example
===>>> Checking jpeg-8_1
	===>>> Updating +REQUIRED_BY
install: /var/db/pkg/jpeg-8_1/+REQUIRED_BY: Permission denied

portmaster should check before it goes through the whole process and
see if it has write access to /var/db/ports.