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/include/defs.h

@@ -134,9 +134,9 @@ struct shared
 	uid_t		euserid;
 	gid_t		groupid;
 	gid_t		egroupid;
-	pid_t		pid;
-	pid_t		ppid;
-	pid_t		current_pid;  /* pid of the current ksh instance */
+	pid_t		pid;		/* $$, the main shell's PID (invariable) */
+	pid_t		ppid;		/* $PPID, the main shell's parent's PID */
+	pid_t		current_pid;	/* ${.sh.pid}, PID of current ksh process (updates when subshell forks) */
 	unsigned char	sigruntime[2];
 	Namval_t	*bltin_nodes;
 	Namval_t	*bltin_cmds;