Have mkerr.pl treat already existing multiline string defs properly

Since source reformat, we ended up with some error reason string
definitions that spanned two lines.  That in itself is fine, but we
sometimes edited them to provide better strings than what could be
automatically determined from the reason macro, for example:

    {ERR_REASON(SSL_R_NO_GOST_CERTIFICATE_SENT_BY_PEER),
     "Peer haven't sent GOST certificate, required for selected ciphersuite"},

However, mkerr.pl didn't treat those two-line definitions right, and
they ended up being retranslated to whatever the macro name would
indicate, for example:

    {ERR_REASON(SSL_R_NO_GOST_CERTIFICATE_SENT_BY_PEER),
     "No gost certificate sent by peer"},

Clearly not what we wanted.  This change fixes this problem.

Reviewed-by: Matt Caswell <matt@openssl.org>