Remove err_fns and associated machinery.

Remove err_fns and associated machinery.

Like all good OpenSSL code, errors was built to be completely extensible.
Thankfully, the ERR_{get,set}_implementation() functions were removed in
r1.127 of err.c, which means that the extensibility can no longer be used.

Take the first of many steps to clean up this code - remove err_fns and
associated machinery, calling functions directly. Rename so that we have
an 'err_' prefix rather than 'int_' (or nothing).

ok joshua@ tb@