Don't report stale signal information for non-signal events in ptrace_lwpinfo.

Once a signal's siginfo was copied to 'td_si' as part of the signal
exchange in issignal(), it was never cleared. This caused future
thread events that are reported as SIGTRAP events without signal
information to report the stale siginfo in 'td_si'. For example, if a
debugger created a new process and used SIGSTOP to stop it after
PT_ATTACH, future system call entry / exit events would set PL_FLAG_SI
with the SIGSTOP siginfo in pl_siginfo. This broke 'catch syscall' in
current versions of gdb as it assumed PL_FLAG_SI with SIGTRAP
indicates a breakpoint or single step trap.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18487