stringcolumns: Make getValue const

They always return pointers into memory which is managed by naemon.
That memory should not be manipulated or freed.

In particular, get_program_version was recently made const for this
reason, and that cascaded into a slew of build errors for livestatus.

Change-Id: I39f4ad8a6712e753f89db6b371084cd7b713409b
Signed-off-by: Robin Sonefors <robin.sonefors@op5.com>