env: Refactor apply into change_ok

Move the read of the old value to inside the check function.  In some
cases it can be avoided all together and at the least the code is only
called from one place.

Also name the function and the callback to more clearly describe what
it does.

Pass the ENTRY instead of just the name for direct access to the whole
data structure.

Pass an enum to the callback that specifies the operation being approved.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

include/environment.h

@@ -188,13 +188,12 @@ int set_default_vars(int nvars, char * const vars[]);
 int env_import(const char *buf, int check);
 
 /*
- * Check if variable "name" can be changed from oldval to newval,
- * and if so, apply the changes (e.g. baudrate).
+ * Check if variable "item" can be changed to newval
  * When (flag & H_FORCE) is set, it does not print out any error
  * message and forces overwriting of write-once variables.
  */
-int env_check_apply(const char *name, const char *oldval,
-			const char *newval, int flag);
+int env_change_ok(const ENTRY *item, const char *newval, enum env_op op,
+	int flag);
 
 #endif /* DO_DEPS_ONLY */