Standardize handling of #ifdef'd options.

Here are the "rules" for handling flags that depend on #ifdef:

- Do not ifdef the enum.  Only ifdef the OPTIONS table.  All ifdef'd
  entries appear at the end; by convention "engine" is last.  This
  ensures that at run-time, the flag will never be recognized/allowed.
  The next two bullets entries are for silencing compiler warnings:
- In the while/switch parsing statement, use #ifdef for the body to
  disable it; leave the "case OPT_xxx:" and "break" statements outside
  the ifdef/ifndef.  See ciphers.c for example.
- If there are multiple options controlled by a single guard, OPT_FOO,
  OPT_BAR, etc., put a an #ifdef around the set, and then do "#else"
  and a series of case labels and a break. See OPENSSL_NO_AES in cms.c
  for example.

Reviewed-by: Matt Caswell <matt@openssl.org>