BUG/MINOR: threads: set at least grp_max when mtpg is too small

BUG/MINOR: threads: set at least grp_max when mtpg is too small

When starting, say, 128 threads with max-threads-per-group set to 2
and MAX_TGROUPS set to the default 32, instead of setting the resulting
number of groups to 32 and threads to 64, they're set to 1 and 32
respectively because the condition to raise grp_min is not satisfied.

Let's cut the condition in two parts to also permit to raise it at
least to grp_max.

This should be backported to 3.2.