[PATCH] Allow bare library names for non-superuser LOAD

Hi,

Non-superusers may use LOAD only for libraries located in
$libdir/plugins, but currently they must specify that directory
explicitly, for example:

LOAD '$libdir/plugins/mylib';

This differs from local_preload_libraries, which accepts a bare library
name and automatically prepends $libdir/plugins.

The attached patch moves this expansion from load_libraries() into
load_file(). As a result, a non-superuser can write:

LOAD 'mylib';

and have it interpreted as:

LOAD '$libdir/plugins/mylib';

Thoughts?

--
Regards,
ChangAo Chen