Rework writing crypto/err/openssl.txt

Reading the prologue of this file conserved the "# Function codes"
line, and then duplicated it when rewriting this file, adding a new
"# Function codes" line everytime there's an update.

Better then to skip over all comment lines and have the prologue
defined in mkerr.pl, just the same as we do with the other affected
files.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3664)