Move NO_SPELL and MAX_SPELL_STUDY from hack.h to spell.h. I didn't
even remember that the latter existed when I put MAX_SPELL_STUDY in the
former recently.
Add a couple of new "spell is almost gone" messages that occur when
casting spells. One matches up with yesterday's change for when you can
re-read a spellbook to reinforce memory of a known spell.
When "too hungry", you can't cast any spells, except detect food so
that you can find something to eat. When "too weak" you couldn't cast any
spells at all; now you can cast restore ability so that you can recover
lost strength. [We still need to come up with a better crowning bonus for
monks than spellbook of restore ability.] I couldn't think of a reasonable
exception for the "can't cast when carrying too much" restriction. If we
can come up with one (levitation?), then remove curse could/should become
an exception for the "no free hands" restriction.
