target: Add Target.revertable_write_value()

Same as write_value(), but returns a context manager that will write
back the old value on exit.

Also add batch_revertable_write_value() that takes a list of kwargs
dict, and will call revertable_write_value() on each of them, returning
a single combined context manager.

doc/target.rst

@@ -346,6 +346,18 @@ Target
        some sysfs entries silently failing to set the written value without
        returning an error code.
 
+.. method:: Target.revertable_write_value(path, value [, verify])
+
+   Same as :meth:`Target.write_value`, but as a context manager that will write
+   back the previous value on exit.
+
+.. method:: Target.batch_revertable_write_value(kwargs_list)
+
+   Calls :meth:`Target.revertable_write_value` with all the keyword arguments
+   dictionary given in the list. This is a convenience method to update
+   multiple files at once, leaving them in their original state on exit. If one
+   write fails, all the already-performed writes will be reverted as well.
+
 .. method:: Target.read_tree_values(path, depth=1, dictcls=dict, [, tar [, decode_unicode [, strip_null_char ]]]):
 
    Read values of all sysfs (or similar) file nodes under ``path``, traversing