vlog: Use OVS_CONSTRUCTOR for vlog initialization

vlog: Use OVS_CONSTRUCTOR for vlog initialization

This allows to get rid of some special segment handling to allow
distributed registering of vlog modules.

Instead use a global list and vlog module constructor functions to
build up the list. That means vlog modules reside within the
compilation unit they are defined in and can be iterated upon
by using the global list vlog_modules.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>