show normal 403 error for persona-idp mismatches

Seems like enough information for users, and slightly better for security. It
also seems like this is hard to do in Apache 2.4's new AAA framework.
make things work with apache-2.4
language/typo fixes in comments/README
try to improve comments a bit
Merge commit 'de67b25dafe0db0e8e963e8b8204f7bdf62f284c'

Conflicts:
	src/mod_authnz_persona.c
Merge commit 'a3c97e81f9b8838628e4bec002fee314ae782474'

Conflicts:
	src/mod_authnz_persona.c
Merge commit 'c7e91c7bdb5f0de33b9e96ff85ec15a17d363d90'

Conflicts:
	src/mod_authnz_persona.c
Merge remote-tracking branch 'upstream/master'