temperature_: fix for F units

Closes #371
assp-message-statistics: fix typo

Closes #167
assp: fix typo

Closes #168
dovecot: Use DERIVE to avoid ugly computations

Using a timed grep with a state computation is usually mostly wrong. The DERIVE
field type is specifically designed for that very purpose.

Closes #264
dovecot: avoid using sort & wc when grepping

egrep has the -c option, that does a wc -l.
Also, there's no need to sort the output prior counting it.
Fix typos in netstat_bsd_m and ejabberd_scanlog
netstat_s: support singular nouns

BSD netstat uses singular nouns when the statistics entry value is 1:
* fragment(s)
* packet(s)
* redirect(s)
* datagram(s)