Fix get_mail_sender_opt() bugs introduced in #175
Three bugs fixed:
1. GNU Mailutils: "-a From: $MAILFROM" (with spaces) was splitting into
separate shell args; restored correct combined form "-aFrom:$MAILFROM".
2. Version capture changed from "2>&1" to "2>/dev/null" in #175,
silently discarding version output for clients that write to stderr;
restored "2>&1" for reliable detection.
3. s-nail: removed "-Sttycharset=latin1" in #175 causes newer s-nail
(Ubuntu 24.04) to fail with "invalid byte ','"; restored the flag
and applied it to the $SDPMAIL==s-nail fallback path as well.
All structural improvements from #175 are preserved: expanded fallback
logic, s-nail/nail checked before numeric version, $SDPMAIL-name
fallbacks, and the else clause for unknown clients.