rename merit fields to be more accurate

[sigilbaram]

I experimented with these fields some more and realized they weren't very well named.

• merits_unlocked -> merits_available This will actually turn back to 0 when switching to a low level job and the menu was greyed out.
• level_capped -> exp_capped has more to do with exp than level. You can be 99 and this will be false until you actual cap your exp.