Check for TERM changes every time we read input.
Also, fix possible crashes when termcap entries aren't found.
Update test-running script to work on GNO (although it seems to hang on some tests)
Reduce stack size to 3k.

Hush uses recursion for (at least) parsing nested constructs like { ... }, so it's possible for pathological/malicious scripts to overflow the stack, but this should be enough for just about anything non-pathological (e.g. up to about 10 levels of nested { }).
Changes to support GNO-style PATH in type and source commands, and in tab completion.
Error handling in case termcap entry can't be found
Enable a couple optimizations that aren't reported to be buggy.