(Special built-in utilities): New node.

(Special built-in utilities): New node.
(printf invocation): builtin -> built-in, for consistency
with POSIX terminology.
(test invocation, pwd invocation):
Use specific rather than generic language to warn about
built-in commands.
(chroot invocation, env invocation, nice invocation, nohup invocation):
Warn that command must not be a special built-in.
(env invocation): Warn about environment variables with unusual
spellings, or duplicates.