Incorrect use of "an" and "a" in code comments and docs

Hi all,

While looking at something else, I have one one occurence of $subject.
Looking more deeply at the code I have found 10 more of them, like:
- sinval is a signal invalidation, so it seems to me that "a" is
correct, not "an".
- I bumped into "an" being used instead of "and" in the GIN code.
- config/c-library.m4 also includes one. If that code is from somewhere
else, fixing this typo would not be appropriate perhaps?

Attached is a proposal of patch to fix all those things.

Thanks,
--
Michael