Several GUCs are simply enumerated strings that are parsed into integer
types behind the scenes. As of 8.4, the GUC system recognizes a new
type, enum, which will do this for us. Move as many as we can to the new
system.

As part of this,
- gp_idf_deduplicate was changed from a char* string to an int, and new
  IDF_DEDUPLICATE_* macros were added for each option
- password_hash_algorithm was changed to an int
- for codegen_optimization_level, "none" is the default now when codegen
  is not enabled during compilation (instead of the empty string).

A couple of GUCs that *could* be represented as enums
(optimizer_minidump, gp_workfile_compress_algorithm) have been
purposefully kept with the prior system because they require the GUC
variable to be something other than an integer anyway.
Signed-off-by: NJacob Champion <pchampion@pivotal.io>