Replace almost all uses of enum connection_state with struct
connection_status. This removes the assumption that errno values used
by the system are between 0 and 100000. The GNU Hurd uses values like
ENOENT = 0x40000002 and EMIG_SERVER_DIED = -308.
This commit is derived from my attachments 450 and 467 to bug 1013.
straconcat reads the args with va_arg(ap, const unsigned char *),
and the NULL macro may have the wrong type (e.g. int).
Many places pass string literals of type char * to straconcat. This
is in principle also a violation, but I'm ignoring it for now because
if it becomes a problem with some C implementation, then so will the
use of unsigned char * with printf "%s", which is so widespread in
ELinks that I'm not going to try fixing it now.
Increment conn->from by the length of the data so that when
abort_connection calls normalize_cache_entry, it doesn't truncate the
cache entry to 0 length.
