Use RTLD_NOW, not RTLD_LAZY, as binding mode for dlopen() on all platforms.

Use RTLD_NOW, not RTLD_LAZY, as binding mode for dlopen() on all platforms.
This restores the Linux behavior to what it was in PG 7.0 and 7.1, and
causes other platforms to agree.  (Other well-tested platforms like HPUX
were doing it this way already.)  Per pghackers discussion over the past
month or so.