Fix install-missing-packages

r519284 introduced a new target, install-missing-packages, which is
meant to allow users to install dependencies via pkg(8) instead of
building them themselves locally.

The target was producing errors when the dependencies were already
available on the system. This commit adjusts this behavior to just
silently do nothing if everything is fine. Also, wrap the command
with SU_CMD for better user experience.

Also, fix some trailing whitespace introduced in the original commit.

Reviewed by: bapt
Approved by: portmgr
Differential Revision: https://reviews.freebsd.org/D23484