Custom extension revision.

Use the same structure for client and server custom extensions.

Add utility functions in new file t1_ext.c.
Use new utility functions to handle custom server and client extensions
and remove a lot of code duplication.
Reviewed-by: Emilia Käsper <emilia@openssl.org>
(cherry picked from commit ecf4d660)

Conflicts:

	ssl/ssl_lib.c
	ssl/ssl_locl.h
	ssl/t1_lib.c