Table data exclusion patch for pg_dump

Hello!

This is a patch that allows choosing not to dump the data for the selected
tables.

The intended usage is to make backups smaller and faster, by allowing skipping
unneeded data, while still generating a backup that can be restored and obtain
a fully working application.

I use it to avoid dumping log data, and the content of tables that keep data
that has a short lifetime (which in the event of a restore would have expired
by then anyway)

The patch adds the -d and -D arguments:
-d --data=TABLE dump data for the named table(s)
-D --exclude-data=TABLE do NOT dump data for the named table(s)

I believe the patch to be complete, though I'm very new to the postgresql
codebase and might have missed something.

The patch applies to HEAD, compiles and worked properly in my tests.