Fix GH-7902: mb_send_mail may delimit headers with LF only

Email headers are supposed to be separated with CRLF. Period.

We introduce a `CRLF` macro for better comprehensibility rig

Fix GH-7902: mb_send_mail may delimit headers with LF only

Email headers are supposed to be separated with CRLF. Period.

We introduce a `CRLF` macro for better comprehensibility right away.

Closes GH-7907.

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