openssl: rename 'errcode_t' to 'sslerr_t'

Turns out "/usr/include/et/com_err.h" typedefs this type (without proper
variable scoping).

comerr is the "common error description library" that apparently might be used
by krb5 code, which then makes this header get used in a curl build.

Reported-by: Bruno Henrique Batista Cruz da Silva
Ref: https://github.com/curl/curl/commit/fc10d860112cfb9057b620fae3e33368008ffdae#commitcomment-100475142