Added shared_memory_locking configuration directive to control mlock(2).

Added shared_memory_locking configuration directive to control mlock(2).

Locking shared memory at startup avoids SIGBUS crashes when kernel runs
out of RAM during runtime. Why not enable it by default? Unfortunately,
locking requires privileges and/or much-higher-than-default
RLIMIT_MEMLOCK limits. Thus, requiring locked memory by default is
likely to cause too many complaints, especially since Squid has not
required that before. The default is off, at least for now.

As we gain more experience, we may try to enable locking by default
while making default locking failures non-fatal and warning about
significant [accumulated] locking delays.