Change functions to pass in a limit rather than calculate it

Some extension handling functions were passing in a pointer to the start
of the data, plus the length in order to calculate the end, rather than
just passing in the end to start with. This change makes things a little
more readable.

Reviewed-by: Emilia Käsper <emilia@openssl.org>

Conflicts:
	ssl/s3_srvr.c
	ssl/ssl_locl.h
	ssl/t1_lib.c