sig_handle_first_stop(): allow NULL leader thread

Remove the ext argument, the ext condition is indicated by NULL td.
If the td argument is NULL, p_xthread is set to NULL, for initial
attach. Previous commit prepared ptrace() code for the case.

sig_suspend_thread() only uses the td arg to avoid interrupting the
current thread as micro-optimization, so it is fine to pass NULL.

Tested by: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D49961