Fix "need root" logic for PORT_DBDIR.

[jonathan]

Summary

When PORT_DBDIR is set to a user directory, we shouldn't need su to
manipulate options. This also shouldn't depend on INSTALL_BY_USER, as
building != installing.

[mat]

head/Mk/bsd.port.mk

5249–5250

It would have been nice to not change the sh style, in the ports/Mk/* we write:

if [ foo ]; then

not

if [ foo ];
then