Modify mount_propagation_flags_from_string to return a normal int code

This means that callers can distiguish an error from flags==0,
and don't have to special-case the empty string.

src/basic/mount-util.c

@@ -693,13 +693,12 @@ int umount_verbose(const char *what) {
 const char *mount_propagation_flags_to_string(unsigned long flags) {
 
         switch (flags & (MS_SHARED|MS_SLAVE|MS_PRIVATE)) {
-
+        case 0:
+                return "";
         case MS_SHARED:
                 return "shared";
-
         case MS_SLAVE:
                 return "slave";
-
         case MS_PRIVATE:
                 return "private";
         }
@@ -707,17 +706,18 @@ const char *mount_propagation_flags_to_string(unsigned long flags) {
         return NULL;
 }
 
-unsigned long mount_propagation_flags_from_string(const char *name) {
+
+int mount_propagation_flags_from_string(const char *name, unsigned long *ret) {
 
         if (isempty(name))
-                return 0;
-
-        if (streq(name, "shared"))
-                return MS_SHARED;
-        if (streq(name, "slave"))
-                return MS_SLAVE;
-        if (streq(name, "private"))
-                return MS_PRIVATE;
-
+                *ret = 0;
+        else if (streq(name, "shared"))
+                *ret = MS_SHARED;
+        else if (streq(name, "slave"))
+                *ret = MS_SLAVE;
+        else if (streq(name, "private"))
+                *ret = MS_PRIVATE;
+        else
+                return -EINVAL;
         return 0;
 }

src/basic/mount-util.h

@@ -63,4 +63,4 @@ int mount_verbose(
 int umount_verbose(const char *where);
 
 const char *mount_propagation_flags_to_string(unsigned long flags);
-unsigned long mount_propagation_flags_from_string(const char *name);
+int mount_propagation_flags_from_string(const char *name, unsigned long *ret);