Clean up temporary-memory management during ispell dictionary loading.

Add explicit initialization and cleanup functions to spell.c, and keep
all working state in the already-existing ISpellDict struct.  This lets us
get rid of a static variable along with some extremely shaky assumptions
about usage of child memory contexts.

This commit is just code beautification and has no impact on functionality
or performance, but it opens the way to a less-grotty implementation of
Pavel's memory-saving hack, which will follow shortly.