New plugin "email" to collectd ham, spam, ... statistics

New plugin "email" to collectd ham, spam, ... statistics

This plugin collects email count and size for each type (e.g. ham, spam,
virus, ...) of emails, spam score values and the count of successful spam
checks (e.g.  BAYES_99, SUBJECT_DRUG_GAP_C, ...).

These information are provided by external programs which communicate with the
plugin thru a UNIX socket and a simple line-based protocol:

/* e-mail type (e.g. ham, spam, virus, ...) and size */
e:<type>:<bytes>

/* spam score */
s:<value>

/* successful spam checks */
c:<type1>[,<type2>,...]

At most MAX_CONNS (currently set to 5) clients can connect to the plugin
simultaneously. Each connection is handled by a separate thread. Any input
line is limited to 256 characters (including the newline character) which
ought to be enough for anybody[tm] by definition. "c"-lines have to be split
up if they grow longer.

Signed-off-by: Sebastian Harl <sh@tokkee.org>