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

doc/src/sgml/pgfreespacemap.sgml

@@ -76,7 +76,7 @@
 
 <screen>
 postgres=# SELECT * FROM pg_freespace('foo');
- blkno | avail 
+ blkno | avail
 -------+-------
      0 |     0
      1 |     0
@@ -101,7 +101,7 @@ postgres=# SELECT * FROM pg_freespace('foo');
 (20 rows)
 
 postgres=# SELECT * FROM pg_freespace('foo', 7);
- pg_freespace 
+ pg_freespace
 --------------
          1216
 (1 row)