Upgraded jobs use wrong pool

With the introduction of a fix for "#1679 Job Overrides are Not listed
In Manual Run Confirmation" we introduced a bug in an interactive run
as we do an pool override early and ignore any changes later on by
setting a suppress boolean value. This doesn't work when a job gets
upgraded later on to lets say Full because no earlier Full was found
but it started out as an incremental Job.

Moved get_level_since_time() from fd_cmds.c to job.c where it
makes much more sense. Fixed comments and let it return a boolean
which is true when it updated the pool due to a missing
full/differential etc. Now when get_level_since_time() returns true
we force apply_pool_overrides() to ignore any overrides and always
reevaluate the pool it needs to save too (fullpool/diffpool/incpool)

Also moved the call to get_level_since_time() and apply_pool_overrides()
into setup_job() so we don't have to code it twice in
do_native_backup_init() and do_ndmp_backup_init().

get_level_since_time() now always encodes the since value into the
jcr->since variable so we only need one argument to the function
being the jcr and no longer a char pointer and size which for most
calls were pointing to something in the jcr anyway.

Fixes #88: Upgraded jobs use wrong pool