Resolve #10435.

Adjust Database{Location} to only attempt to create a mimetype when
actually necessary, and fail otherwise if a writable version doesn't yet
exist. Correspondingly, adjust callers such as
DatabaseLocation::DeleteAttribute(). Fixes a problem where a caller asking
to perform a mimeset could fail early due to SetSupportedTypes() attempting
to update the read-only mime database entry supplied by a package, and
consequently most of the mimeset operations would be skipped.

headers/private/storage/mime/DatabaseLocation.h

@@ -38,9 +38,9 @@ public:
 			// opening type nodes
 
 			status_t			OpenType(const char* type, BNode& _node) const;
-			status_t			OpenOrCreateType(const char* type,
-									BNode& _node, bool* _didCreate = NULL)
-									const;
+			status_t			OpenWritableType(const char* type,
+									BNode& _node, bool create,
+									bool* _didCreate = NULL) const;
 
 			// generic type attributes access