hexdump: revert global exitval variable change

The change f2a037fb7b153954d5d34cca48182b6d8832fcfa had unfavorable
effect of making hexdump to return non-zero exit value always.

This happen because oversight when 'exitval' gets to be set.  By clance,
one might expect main() to call next() which will return value for
'exitval'.  That assessment misses later call chain main() -> display()
-> get() -> next(), which in reverse should return correct value for
'exitval'.

It was mentioned in util-linux maillist that Ondrej Oprala is working on
making hexdump less insane.  That in mind it seems best to simply to
revert the global 'exitval' change rather than conflicting with Ondrej's
work by adding new function arguments, return values, etc.  Besides
fixing the current hexdump is very likely throw away effort.

Reference: http://markmail.org/message/sbnvuhkboreujj5p
Reported-by: Dave Reisner <d@falconindy.com>
CC: Ondrej Oprala <ooprala@redhat.com>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>