Change `load' to interact better with Emacs interface. Dissect REP

Change `load' to interact better with Emacs interface.  Dissect REP
loop slightly to allow its parts to be used independently by `load'.
Now whenever `load' prints a value it is also entered in the REP
printer history.  Also, the value of the last file loaded is returned
to the REP loop as the value of the `load' expression.

All of this is useful for allowing Emacs to use `load' for zapping.
Now, zapping from a file is more or less equivalent to zapping through
a pipe.  Before, there were significant differences, especially
noticeable in interaction with the printer history.