Make struct grouptask gt_name member a char array

Previously, it was just a pointer which was copied, but
some callers pass in a stack variable which will go out of scope.
Add GROUPTASK_NAMELEN macro (32) and snprintf() the name into it,
using "grouptask" if name is NULL. We can now safely include
gtask->gt_name in console messages.

Reviewed by: sbruno
Approved by: sbruno (mentor)
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D12449