Allow history file and size to be configurable.

Up until now the readline history file always had a fixed name and size
of 100 entries in memory and 200 on disk. You could always configure a
name for the history file but it was never used. We now also always
truncate the history size to the configured size before reading it into
memory.
First attempt at proper support for JS_Warnings

Hopefully fixes a TODO from March 2009 when the first attempt was made
to introduce JS_Warnings but things never finished.

Fixes #255: Bad mail subject when an backup terminates with warnings
Change getJobType() == JT_XX into is_JobType(JT_XX)