[PATCH] Extend the length of BackgroundWorker.bgw_library_name

Hi,

I want to suggest a patch against master (it may also be worth backporting
it) that makes it possible to use longer filenames (such as those with
absolute paths) in `BackgroundWorker.bgw_library_name`.

`BackgroundWorker.bgw_library_name` currently allows names up to
BGW_MAXLEN-1, which is generally sufficient if `$libdir` expansion is used.

However, there are use cases where [potentially] longer names are
expected/desired; for example, test benches (where library files may not
[or can not] be copied to Postgres installation) or alternative library
installation methods that do not put them into $libdir.

The patch is backwards-compatible and ensures that bgw_library_name stays
*at least* as long as BGW_MAXLEN. Existing external code that uses
BGW_MAXLEN is a length boundary (for example, in `strncpy`) will continue
to work as expected.

The trade-off of this patch is that the `BackgroundWorker` structure
becomes larger. From my perspective, this is a reasonable cost (less than a
kilobyte of extra space per worker).

The patch builds and `make check` succeeds.

Any feedback is welcome!

-- 
http://omnigres.org
Yurii