Pull the PT_ATTACH case out of the 'sendsig:' block.

Most of the conditionals in the 'sendsig:' block are now only different
for PT_ATTACH vs other continue requests. Pull the PT_ATTACH-specific
logic up into the PT_ATTACH case and simplify the 'sendsig:' block. This
also permits moving the unlock of proctree_lock above the sendsig: label
since PT_KILL doesn't hold the lock and and the other cases all fall
through to the label.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D13073