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.

Also changed all validators, which accept None to check strings
without case ("none" vs "None").

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

Closes: #2543
Merge pull request #2555 from JanSchulz/bug_2543

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