mx_call_external: Change EPIPE to EPROTO

If an external helper does something unexpected like exiting with a
non-zero exit status or sending more data than mxqd wants to handle,
mx_call_external sets errno to an error code.

Currenty we use EPIPE ("Broken pipe"). Change this to EPROTO ("Protocol
error"), which seems to be better fitting.

We assume that the external helper, which shares its stderr with mxqd,
has already sent some diagnostic to the logfile.

mx_util.c

@@ -1360,7 +1360,7 @@ static char *mx_call_external_v(char *helper, char **argv) {
         goto err_close;
     }
     if (len == sizeof(buf)) {
-        err = EPIPE;
+        err = EPROTO;
         goto err_close;
     }
 
@@ -1369,7 +1369,7 @@ static char *mx_call_external_v(char *helper, char **argv) {
     int wstatus;
     waitpid(pid, &wstatus, 0);
     if (wstatus != 0) {
-        err = EPIPE;
+        err = EPROTO;
         goto err_err;
     }
     buf[len] = '\0';

test_mx_util.c

@@ -545,13 +545,13 @@ static void test_mx_call_external() {
     assert(errno == 999);
 
     line = mx_call_external("/usr/bin/false", NULL);
-    assert(line == NULL && errno==EPIPE);
+    assert(line == NULL && errno==EPROTO);
 
     line = mx_call_external("/usr/bin/cat", "/usr/bin/bash", NULL);
-    assert(line == NULL && errno==EPIPE);
+    assert(line == NULL && errno==EPROTO);
 
     line = mx_call_external("/usr/bin/yes", NULL);
-    assert(line == NULL && errno==EPIPE);
+    assert(line == NULL && errno==EPROTO);
 
 }