make sure that pushed-back 0xff character isn't accidentally
interpreted as an EOF because of sign extension when chars were being
assigned to pbents. (signed-char machines lost.) To do this, make
putback() and pbstr() operate only on unsigned chars, and add a
putbackeof() function to do the obvious thing when necessary.
that typedef 'short'. 'char' (which was previously used) because char
may be unsigned and ((char)EOF) != EOF if that is the case. That was
causing the (char)EOF (0xff) pushed back in main to be interepreted as
a character, and, in some cases, to be written to the output. 'short'
was used rather than 'signed char' because if the latter is used,
0xff characters in the input would confuse m4. (No point in introducing
(more?) 8-bit lossage.)
