Make it possible to add mpl.rcParams to itself or deepcopy

Before it was not possible to set all rcParams to mpl.rcParams or use
 copy.deepcopy on mpl.rcParams.

There were two problems:
 1. validate_bool_maybe_none(None) was raising ValueError
 -> fixed by accepting this value
 2. svg.embed_char_paths was deprecated by svg.fonttype but both have
 different valid values (bool vs strings).
 -> fixed by adding a "value translation" lambda to deprecatedmap
 which translates the boolean values to the string values of
 svg.fonttype.

I also added a RcParam.update() method so that these values are also
validated.

Test for this: mpl.tests.test_rcparams.test_Bug_2543()

Closes: #2543