Restored Squid ability to cache (in memory) when no disk caches are configured

Restored Squid ability to cache (in memory) when no disk caches are configured
which was lost during r12662 "Bug 3686: cache_dir max-size default fails"

The bug was hidden until the memory cache code started calling
StoreEntry::checkCachable() in branch r13315, exposing the entry size check to
a broken limit.

r12662 converted store_maxobjsize from a sometimes present disk-only limit to
an always set Squid-global limit. However, store_maxobjsize value was only
calculated when parsing cache_dirs. A config without cache_dirs would leave
store_maxobjsize at -1, triggering "StoreEntry::checkCachable: NO: too big"
prohibition for all responses.

This change moves store_maxobjsize calculation from parser to storeConfigure()
where some other Store globals are computed after squid.conf parsing.

Also honored memory cache size limit (just in case cache_mem is smaller than
maximum_object_size_in_memory) and removed leftover checks for
store_maxobjsize being set (it should always be set, at least to zero).