redirection from builtins without forking

[Paul Haahr <haahr@mv.us.adobe.com>]

a prototype of the mechanism i described in my last note on the subject
seems to work ok.  it will ship in .75; i'd release it now except that
it's a pervasive change and i'm sure i haven't found every place that
needs to be cognizant that a file descriptor isn't just a file descriptor
any more.

the upshot:  no funny new options to echo, no new output command, just use
redirections.  and, even John will approve of the behaviour  :-)