ERR_ cleanup

Remove ERR_[gs]et_implementation as they were not undocumented and
useless (the data structure was opaque).

Halve the number of lock/unlock calls in almost all ERR_
functions by letting the caller of get_hash or int_thread_set
able to lock.  Very useful when looping, such as adding errors,
or when getting the hash and immediately doing a lookup on it.

Reviewed-by: Richard Levitte <levitte@openssl.org>