system_config docs confusing

[josephholsten]

The docs for the system config in sqitch-config(3) and lib/App/Sqitch/Config.pm:120 are a bit confusing and would probably be improved by making the doc more similar.

In particular, the string $($etc_prefix)/sqitch.conf found in sqitch-config(2), sqitch-add(3) seems confusing, I expected to set /etc/sqitch.conf, /usr/etc/sqitch.conf, or /usr/local/etc/sqitch.conf. And the strings $(etc_path)/sqitch.conf & ($etc_path)/sqitch.conf are equally confusing.

Personally, I'd like:

• to use the GNU Coding Standard terms for the path, so either $(prefix)/etc/sqitch/sqitch.conf or $(sysconfdir)/sqitch/sqitch.conf if you must. I'd even be ecstatic for it to be consistently documented as $Config{prefix}/etc/sqitch/sqitch.conf
• to have a way to find out what the prefix baked in is, perhaps merely by printing the system config file [Edit: just found --etc-path, which is sad since I only noticed it after reading the source, not the manual. Is there a more common name for this? Would it just be worth pointing out in the sqitch-config(3) doc?]

Anyway, thanks so much for the awesome tool! I'll gladly take a stab at a PR if you'd like, but my perl has been rusting on the shelf for about a decade. Hopefully it would just be doc changes.

[theory]

Pull request would be great.

--etc-path, BTW, is documented in man sqitch, but it should probably be mentioned anywhere we use $etc_path or $prefix or whatever.