implement keep_active=error to preserve shepherd files

[dlove]

@type enhancement

Make execd param keep_active=error preserve the active_jobs directory in
case of failed jobs (SGE failure, rather than non-zero condition code
from job). See
​http://gridengine.eu/grid-engine-internals/103-univa-grid-engine-81-features-part-3-simplified-debugging-in-case-of-failures-2012-05-30
It's probably good enough to leave the files in place, especially on a
shared spool. Assume they'll be cleared by an execd restart.

I think it needs an extension of the parameter parsing for string
values.