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/sh/main.c

@@ -146,8 +146,6 @@ int sh_main(int ac, char *av[], Shinit_f userinit)
 	{
 		/* begin script execution here */
 		sh_reinit((char**)0);
-		shp->gd->pid = getpid();
-		shp->gd->ppid = getppid();
 	}
 	shp->fn_depth = shp->dot_depth = 0;
 	command = error_info.id;
@@ -518,7 +516,7 @@ static void	exfile(register Shell_t *shp, register Sfio_t *iop,register int fno)
 			{
 				buff.mode = SH_JMPERREXIT;
 #ifdef DEBUG
-				errormsg(SH_DICT,ERROR_warn(0),"%d: mode changed to JMP_EXIT",getpid());
+				errormsg(SH_DICT,ERROR_warn(0),"%d: mode changed to JMP_EXIT",shgd->current_pid);
 #endif
 			}
 		}