Correct two errors which sabotaged venti's calculation of default cache sizes:
- stfree is uninitialized if '-m' parameter not given, which causes
the first call of allocbypcnt() to produce bogus results
- config.mem is set to ~0 if 'mem =' not given in config file, but
sizeallocs() takes this as a genuine value and overrides the default
with it, and then allocminima() overrides it again to 1MB