rm redundant getpid(2) syscalls (re: 9de65210)

Now that we have ${.sh.pid} a.k.a. shgd->current_pid, which is
updated using getpid() whenever forking a new process, there is no
need for anything else to ever call getpid(); we can use the stored
value instead. There were a lot of these syscalls kicking around,
some of them in performance-sensitive places.

The following lists only changes *other* than changing getpid() to
shgd->currentpid.

src/cmd/ksh93/include/defs.h:
- Comments: clarify what shgd->{pid,ppid,current_pid} are for.

src/cmd/ksh93/sh/main.c,
src/cmd/ksh93/sh/init.c:
- On reinit for a new script, update shgd->{pid,ppid,current_pid}
  in the sh_reinit() function itself instead of calling sh_reinit()
  from sh_main() and then updating those immediately after that
  call. It just makes more sense this way. Nothing else ever calls
  sh_reinit() so there are no side effects.

src/cmd/ksh93/sh/xec.c: _sh_fork():
- Update shgd->current_pid in the child early, so that the rest of
  the function can use it instead of calling getpid() again.
- Remove reassignment of SH_PIDNOD->nvalue.lp value pointer to
  shgd->current_pid (which makes ${.sh.pid} work in the shell).
  It's constant and was already set on init.

src/cmd/ksh93/edit/history.c

@@ -363,7 +363,7 @@ retry:
 	if(hist_clean(fd) && hist_start>1 && hsize > HIST_MAX)
 	{
 #ifdef DEBUG
-		sfprintf(sfstderr,"%d: hist_trim hsize=%d\n",getpid(),hsize);
+		sfprintf(sfstderr,"%d: hist_trim hsize=%d\n",shgd->current_pid,hsize);
 		sfsync(sfstderr);
 #endif /* DEBUG */
 		hp = hist_trim(hp,(int)hp->histind-maxlines);