update maint.mk; fix syntax-check failures

[Jim Meyering]

I noticed that autoconf's maint.mk was out of date wrt gnulib's,
so updated it.  Also, even before the update, "make syntax-check"
was failing numerous tests.  With the update, even more (added tests)
rules were failing.  With this series, I've addressed some of the
issues and arranged to skip the remaining failing tests, so that now
"make syntax-check" succeeds once again.

Feel free to remove entries from cfg.mk's list of skipped
tests and address the failures, if you think it's appropriate.


[1/7] doc: fix grammar/doubled-word errors
[2/7] maint: remove empty lines at EOF
[3/7] maint: tweak to avoid triggering space-tab-prohibiting
[4/7] maint: placate syntax-check rules: exempt some false
[5/7] maint.mk: update from gnulib
[6/7] maint: also sync maint.mk and useless-if-before-free
[7/7] maint: fix or disable failing syntax-check rules