Yet another round of accumulated tweaks and cleanups

Notable changes:

- sh/timers.c: Rename timerdel() to sh_timerdel() for consistency;
  we had timerdel() but sh_timeradd().

- include/shell.h: Remove unused sh struct members:
  - sh.lastpath (this was still being saved/restored in several
    places, but not actually used since 2008-06-02 ksh93t-)
  - sh.lastbase (unused since 2000-10-31 ksh93k)
  - sh.inpool (unused since libcoshell was removed in 3613da42)

- sh/xec.c: sh_exec(): Add comments for the various command types.

src/cmd/ksh93/edit/edit.c

@@ -211,7 +211,7 @@ int tty_set(int fd, int action, struct termios *tty)
 /*{	TTY_COOKED( fd )
  *
  *	This routine will set the tty in cooked mode.
- *	It is also called by error.done().
+ *	It is also called by sh_done().
  *
 }*/
 

src/cmd/ksh93/edit/vi.c

@@ -193,10 +193,10 @@ static int	search(Vi_t*,int);
 static void	sync_cursor(Vi_t*);
 static int	textmod(Vi_t*,int,int);
 
-/*+	VI_READ( fd, shbuf, nchar )
+/*+	ED_VIREAD( context, fd, shbuf, nchar, reedit )
  *
  *	This routine implements a one line version of vi and is
- * called by _filbuf.c
+ * called by slowread() in io.c
  *
 -*/