Rationale:
SIGURG is configured by ftpd to interrupt system calls, which is useful
during data transfers. However, SIGURG could interrupt I/O on the
control channel as well, which was mistaken for the end of the session.
A practical example could be aborting the download of a tiny file,
when the abort sequence reached ftpd after ftpd had passed the file
data to the system and returned to its command loop.
Reported by: ceri
MFC after: 1 week
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