BUG-1665: rework prefix assignment logic

Expand the charset, but do not include xXmMlL in it, preventing a
possible issue. Also speed up the codec process by having things aligned
at 32, allowing us to use shift of 5.

Change-Id: I812b24afc00e3711ae38a69144d4b15fa3232253
Signed-off-by: Robert Varga <rovarga@cisco.com>