Use grep's --null option (Bug#6843)

Use grep's --null option (Bug#6843)

* lisp/progmodes/grep.el (grep-use-null-filename-separator): New option.
(grep--regexp-alist-column, grep--regexp-alist-bin-matcher)
(grep-with-null-regexp-alist, grep-fallback-regexp-alist): New
constants, replacing `grep-regexp-alist'.
(grep-regex-alist): Mark the variable obsolete, add a new function of
the same name to replace it.
(grep-compute-defaults): Compute default for
`grep-use-null-filename-separator'.
(grep-mode): Set compilation-error-regexp-alist (buffer locally) to the
value of `grep-with-null-regexp-alist' or `grep-fallback-regexp-alist'
according to `grep-use-null-filename-separator'.
* lisp/progmodes/xref.el (xref-collect-matches): Call
`grep-regex-alist' instead of the obsolete variable.  Don't hardcode
grep-regexp-alist match groups.
* etc/NEWS: Announce new use of --null.  Move 'grep-save-buffers'
item under "Grep" heading as well.