Implement and use a do_cipher() wrapper

Implement and use a do_cipher() wrapper

Instead of using five different idioms for eight callers of the do_cipher()
method in EVP_{Decrypt,Encrypt}{Update,Final_ex}(), wrap the API insanity
in an evp_cipher() function that calls do_cipher() as appropriate depending
on the EVP_CIPH_FLAG_CUSTOM_CIPHER being set or not. This wrapper has the
usual OpenSSL calling conventions.

There is one complication in EVP_EncryptUpdate() in the case a previous
call wrote only a partial buffer. In that case, the evp_cipher() call is
made twice and the lengths have to be added. Add overflow checks and only
set outl (the number of bytes written) to out on success.

ok jsing