unify mkheader, mkdatadefs scripts

[Jeff Johnson]

The two header generation scripts (build_env/headers/ mkheader and mkdatadefs)
share many common traits - they are outputting header-style data. They are not
identical: mkdatadefs produces output only for one specific LSB
version/architecture pair (this is the material used in the LSB specification
under "Data Definitions"), which mkheader has to deal with all LSB
versions/architectures through #ifdefs, and also adds some of the wrapping and
protection mechanisms to make it a full header which can be used in code
compilation. Nonetheless, there's clearly a common part, and when we sometimes
find differences it suggests that they should share a common code piece where
possible, so the two scripts don't change out of sync. This has been discussed
in the past, and had a mention in bug 2473#c2

And example of differing behavior is in bug 3832, where we see the upstream
construct

typedef struct StructName {
  ...
} StructRef;

emitted differently by mkheader and mkdatadef.

Note: this bug was requested during bug triage 15 Aug to record the request.