Remove trailing whitespace from *.sgml files.

Remove trailing whitespace from *.sgml files.

Historically we've been lax about this, but seeing that we're not
lax in C files, there doesn't seem to be a good reason to be so
in the documentation.  Remove the existing occurrences (mostly
though not entirely in copied-n-pasted psql output), and modify
.gitattributes so that "git diff --check" will warn about future
cases.

While at it, add *.pm to the set of extensions .gitattributes
knows about, and remove some obsolete entries for files that
we don't have in the tree anymore.

Per followup discussion of commit 5a892c9b1.

Discussion: https://postgr.es/m/E1nfcV1-000kOR-E5@gemulon.postgresql.org