channel - Closed channel stream to retrieve exit signal from.
exitsignal - If not NULL, is populated by reference with the exit signal (without leading "SIG"). Note
that the string is stored in a newly allocated buffer. If the remote program exited cleanly, the
referenced string pointer will be set to NULL.
exitsignal_len - If not NULL, is populated by reference with the length of exitsignal.
errmsg - If not NULL, is populated by reference with the error message (if provided by remote server, if
not it will be set to NULL). Note that the string is stored in a newly allocated buffer.
errmsg_len - If not NULL, is populated by reference with the length of errmsg.
langtag - If not NULL, is populated by reference with the language tag (if provided by remote server, if
not it will be set to NULL). Note that the string is stored in a newly allocated buffer.
langtag_len - If not NULL, is populated by reference with the length of langtag.