addrAppend() joined all addresses onto one line, so a To, Cc, Bcc or
Reply-To header with many recipients could go past the 998 character
limit in RFC 5322. Fold between addresses when the next one would not
fit. Short lists are unchanged.
Fixes#2690
openssl_pkcs7_sign() returns its headers and body with bare LF line
breaks. They were copied into the message verbatim, so signed mail was
rejected by SMTP servers that enforce RFC 5321 line endings with
550 5.6.11 SMTPSEND.BareLinefeedsAreIllegal.
Fixes#3067
dualfroz