dlopen: sanity check the DL directory

If the user provides $FI_EXTDIR and it either doesn't exist or isn't a
directory, provide a warning message.  If the compiled-in default DL
directory doesn't exist or isn't a directroy, silently skip it.

Also remove a debugging FI_WARN for when we detect static linking.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>