Only clear a pending thread event if one is pending.

This fixes a panic when attaching to an already-stopped process after
r325028. While here, clean up a few other things in the control flow
of the 'sendsig' section:

• Only check for P_STOPPED_TRACE rather than either of P_STOPPED_SIG or P_STOPPED_TRACE for most ptrace requests. The signal handling code in kern_sig.c never sets just P_STOPPED_SIG for a traced process, so if P_STOPPED_SIG is stopped, P_STOPPED_TRACE should be set anyway. Remove a related debug printf. Assuming P_STOPPED_TRACE permits simplifications in the 'sendsig:' block.
• Move the block to clear the pending thread state up into a new block conditional on P_STOPPED_TRACE and handle delivering pending signals to the reporting thread and clearing the reporting thread's state in this block.
• Consolidate case to send a signal to the process in a single case for PT_ATTACH. The only case that could have been in the else before was a PT_ATTACH where P_STOPPED_SIG was not set, so both instances of kern_psignal() collapse down to just PT_ATTACH.

Reported by: pho, mmel
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D12837