Some of the statistics we gather are accumulated totals, while others are

Some of the statistics we gather are accumulated totals, while others are
instantaneous measurements.  For instance, we report the total number of
allocator requests made over the child's lifetime, but we also report the
amount of storage in use at any particular moment.

The difference is important, because accumulated totals can be averaged
over the program's lifetime (or over the last N seconds), but instantaneous
measurements can't.

Recycle the format field in MAC_STAT() (it was never used anyway) into a
single-character flag indicating whether each item is an accumulated total
('a') or an instantaneous measure ('i').  Use this in varnishstat to skip
averaging non-averageable numbers.

Also rework varnishstat's "once" mode to show 1) each statistic's symbolic
name, 2) its current value, 3) if appropriate, its value averaged over the
process lifetime, and 4) its description.

The reason for displaying the symbolic name is to simplify scripting, and
to serve as a reference for looking up symbolic names to pass to e.g. the
upcoming Nagios plugin.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1587 d4fa192b-c00b-0410-8231-f00ffab90ce4