Add linear m4_bpatsubsts for m4 1.4.x.

Add linear m4_bpatsubsts for m4 1.4.x.

* lib/m4sugar/m4sugar.m4 (m4_bpatsubsts): Match documentation
about anchors, even for only one substitution.
* lib/m4sugar/foreach.m4 (_m4_bpatsubsts): New implementation.
* doc/autoconf.texi (Conditional constructs) <m4_bpatsubsts>:
Clarify behavior with regard to quoting.
* tests/m4sugar.at (recursion): Test scaling of m4_bpatsubsts.
(m4@&t@_bpatsubsts): New test.
* NEWS: Document the linear guarantee.

Signed-off-by: Eric Blake <ebb9@byu.net>