Rework EVP_PKEY_set_type{,_str}()

Rework EVP_PKEY_set_type{,_str}()

These two functions previously wrapped a pkey_set_type() helper, which
was an utter mess because of ENGINE. With the long awaited departure of
ENGINE, this function became a lot simpler. A further simplification is
obtained by not doing the optimization to avoid an ameth lookup: this
requires walking a list of 11 ameths. We should consider bsearch()...

With this gone and a saner implementation of EVP_PKEY_free_it(), we can
implement these functions with a dozen lines of code each.

ok jsing