Improve shell documentation:

* Consistently misspell built-in as builtin.

* Add a builtin(1) manpage and create builtin(1) MLINKS for all shell
  builtin commands for which no standalone utility exists.  These MLINKS
  replace those that were created for csh(1).

* Add appropriate xrefs for builtin(1) to the csh(1) and sh(1) manpages,
  as well as to the manpages of standalone utilities which are supported
  as shell builtin commands in at least one of the shells. In such
  manpages, explain that similar functionality may be provided as a
  shell builtin command.

* Improve sh(1)'s description of the cd builtin command. Csh(1) already
  describes it adequately. Replace the cd(1) manpage with a builtin(1)
  MLINKS link.

* Clean up some mdoc problems: use Xr instead of literal "foo(n)"; use
  Ic instead of Xr for shell builtin commands.

* Undo English contractions.

Reviewed by:	mpp, rgrimes

bin/csh/Makefile

@@ -15,10 +15,10 @@ SRCS=	alloc.c char.c const.c csh.c dir.c dol.c err.c exec.c exp.c file.c \
 .PATH:	${.CURDIR}/../../usr.bin/printf
 
 MAN1=	csh.1
-MLINKS=	csh.1 limit.1 csh.1 alias.1 csh.1 bg.1 csh.1 dirs.1 csh.1 fg.1 \
-	csh.1 foreach.1 csh.1 history.1 csh.1 jobs.1 csh.1 popd.1 \
-	csh.1 pushd.1 csh.1 rehash.1 csh.1 repeat.1 csh.1 suspend.1 \
-	csh.1 stop.1 csh.1 source.1
+# MLINKS for Shell built in commands for which there are no userland
+# utilities of the same name are handled with the associated manpage,
+# builtin.1 in share/man/man1/.
+
 CLEANFILES+=const.h errnum.h
 
 const.h: errnum.h