Clear the single-step flag for signal handlers. This fixes bogus trace

Clear the single-step flag for signal handlers.  This fixes bogus trace
traps on the first instruction of signal handlers.

In trap.c:syscall(), fake a trace trap if the single-step flag was set
on entry to the kernel, not if it will be set on exit from the kernel.
This fixes bogus trace traps after the last instruction of signal handlers.

gdb-4.18 (the version in FreeBSD) still has problems with the program in
the PR.  These seem to be due to bugs in gdb and not in FreeBSD, and are
fixed in gdb-5.1 (the distribution version).

PR:		33262
Tested by:	k Macy <kip_macy@yahoo.com>
MFC after:	1 day