Standardize root field type prefixes

Standardize root field type prefixes

This patch changes `cgen.py` and what depends on it so that:

* `_RootFtPrefixes` attribute names always begin with `P` (packet) or
  `ER` (event record).

* `_RootFtPrefixes` attribute values for event record prefixes do not
  begin with `er`.

  The C source context where this is used indicates that it's an event
  record field, so `er` is redundant.

  This also makes the documentation easier to understand.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>