Categorize level-0/1 messages; part 1: "easy" problem messages (#946)

Categorize level-0/1 messages; part 1: "easy" problem messages (#946)

Today, it is impractical to reliably identify important cache.log
messages in automated sysadmin notification systems. No regular
expression is guaranteed to match all important messages and none of the
unimportant ones. While message importance is naturally subjective (with
cache_log_message and notification system regexes available to customize
handling of specific messages), even reliable detection of _potentially_
important messages (to bootstrap admin customization efforts) is
impractical today. Any notification configuration is likely to miss an
important message (in today's or future code) and/or spam admins with
unimportant messages.

Fully addressing this problem requires many changes. This change lays a
foundation for that work by standardizing three well-known message
prefixes (`FATAL:`, `ERROR:`, and `WARNING:`) for debugs() messages
reporting various problems. Eventually, all problem-reporting messages
(and only those) will use one of the standard prefixes. This change
converts 300+ debugs(), bringing the total to 900+ messages using a
a standard prefix. There are more than 5'000 debugs() in Squid.

The scope of this change are problem-reporting debugs() statements that
can be detected and converted by a simple script. We also applied a few
easy-to-automate polishing touches to modified debugs(), like replacing
debug level 0 with DBG_CRITICAL, expanding some contractions (e.g.,
"can't"), and fixing grammar (e.g., "is has").