Add timestamps to (most) FATAL messages (#179)

Add timestamps to (most) FATAL messages (#179)

Reliable timestamp information is often critical for triage. We can use
the existing debugs() interface to add timestamps to FATAL messages. The
affected code already calls such risky functions as
storeDirWriteCleanLogs() so calling debugs() instead of printing
directly to files/syslog should not make things worse.

FATAL messages that were also logged to syslog (at LOG_ALERT level) are
still logged to syslog (at that same level, but now with the usual
Squid-generated prefix). Such syslog alerts can now be easily triggered
via a new ForceAlert() API.

Also treat segmentation faults, bus errors, and other signal-based
sudden deaths the same as most other FATAL errors -- log them to syslog.

This is a Measurement Factory project.