ports-mgmt/synth: Fix two significant bugs

The big one: FreeBSD distfiles are located by default inside the ports
tree (which is kind of insane). I forgot this and people that did not
modify DISTDIR had this directory full of files, which synth interprets
as non-functioning ports. Make directory scans skip this directory if
found to correct.

The smaller one: Synth was supposed to squawk if it couldn't find a
valid portsdir at the environment (PORTSDIR), /usr/dports, or /usr/ports.
The squawk didn't work. Moreover, it wouldn't check PORTSDIR set at
/etc/make.conf. This change makes synth check there after the env.
check (thank you for the hint, bapt@). It also correctly squawks
without creating synth.ini when PORTSDIR isn't valid.