Fix SA_SIGINFO | SA_RESETHAND handling. The sysent' sv_sendsig()
method needs pre-reset state of the ps_siginfo to correctly construct
signal frame.

Move sigdflt() call after the sv_sendsig() invocation in postsig().
Simultaneously extract common code from trapsignal() and postsig()
into new helper postsig_done().

Submitted by: rea
MFC after: 1 week