maint-mode: fix botched configure messages

maint-mode: fix botched configure messages

This change fixes automake bug#9890.

* m4/maintainer.m4 (AM_MAINTAINER_MODE): Fix the "checking ..."
configure message related to the enabling/disabling of maintainer
mode.
* tests/help-maintainer.test: Renamed ...
* tests/maintmode-configure-msg.test: ... to this, and extended
to cover the fixed bug.  Also, since we are at it, make grepping
of configure help screens stricter, and throw in some few other
changes to reduce code duplication and enhance readability.
* tests/Makefile.am (TESTS): Update.
* THANKS: Update with Zack's new e-mail address.

Report and initial patch by Zack Weinberg, test cases added by
Stefano Lattarini.