ptrace(2): expand ability to fetch syscall parameters

Do not limit lwpinfo reporting of syscall number and args to SCE/SCX
events. When td_sa holds the values, we can report them. Clear
td_sa.code in TDA_SIG ast handler: this handler is run when the process
is traced, and it is run with the last ptracestop() points before the
return to userspace.

This allows debugger to infer the interrupted syscall immediately after
PT_ATTACH without the need to loose control to the debuggee' thread. It
should work even when the debuggee is stopped in AST.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D49430