Bug 3449: shm_open failed (part 4: fixing memory_cache_shared defaults)

Bug 3449: shm_open failed (part 4: fixing memory_cache_shared defaults)

Properly initialize memory_cache_shared when not explicitly configured and
do not allocate shared memory segments if memory_cache_shared is off.

Prior to this change, shared memory was requested (during rrClaimMemoryNeeds)
before memory_cache_shared was initialized (during rrAfterConfig, unless
explicitly set in squid.conf). Moreover, MemStore::EntryLimit() calculation
ignored memory_cache_shared setting. All that resulted in an attempt to create
shared memory segments where none were needed or possible.

We now have a new Runners Registry (rrFinalizeConfig) that is dedicated to
finalizing complex configuration options, before those configurations options
are used to initialize modules, features, and services. As far as shared
memory is concerned, the initialization order is now:

  * rrFinalizeConfig: finalize memory_cache_shared if needed
  * rrClaimMemoryNeeds: request shared memory pages for the caches
  * rrAfterConfig: create shared memory segments

The same bug 3449 also needed a fix for shared segment paths (trunk r11961).