grep: add --null flag

grep: add --null flag

makes grep print an ASCII NUL byte after the file name to make the
output unambiguous even in the presence of file names with funny
characters.

A previous iteration of this diff was improved by benno@ and tedu@ and
discussed also with sthen@ and deraadt@.  deraadt@ also improved the
manpage changes in this version of the diff.

OK deraadt@