We now have several things that are stored in cache:

1. The script's module, stap.c and stap.ko
2. The common autoconf result, stapconf.h
3. The tracepoint query modules, tracequery.ko
4. The @cast w/ header modules, typequery_umod.so and typequery_kmod.ko

We have a single session variable, use_cache, which controls all of these.  When
someone uses the -k or -m options, we end up disabling all of the cache.  We
really only need to disable #1 in that case, and it would be nice to still
benefit from the rest of the caching.

While we're at it, we could use some command-line options to control the cache
for debugging.  A few ideas:
  --disable-cache : turn off all caching
  --clean-cache   : clean up stale entries and then quit
  --poison-cache  : force regeneration of items that would have hit the cache