* 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
f106663eef