Generate GUC tables from .dat file

My idea is to store the information in guc_tables.c in a .dat file 
similar to the catalog data in src/include/catalog/, and generated 
guc_tables.c from that.  I want to make it easier to edit that 
information, and I want to be able to make changes to the downstream 
data structures more easily.  (Essentially, those are the same reasons 
as for the original adoption of the .dat format.)

An important project is to adapt the GUC system to a multithreaded 
server.  The leading idea is to convert most global variables to 
thread-local storage.  But this doesn't work with the current global 
structs, because they can't contain a pointer to a thread-local 
variable.  Some workarounds and changes exist in WIP threading branches, 
but they all require at least some mechanical changes to the global 
tables.  If these tables could be generated automatically, it would be 
easier to try out different things.

More generally, maybe the current format of a global struct that points 
to many global variables is not the right one anymore.  Maybe the global 
variables should be packaged into a struct, or several structs.  Or 
maybe the whole thing could just be a hash table and you retrieve values 
when you need them. Who knows.  But this would make it easier to 
experiment and make changes.

Another possible benefit would be that we could generate the 
postgresql.conf.sample file.  This is also very tedious to edit and 
maintain, and sometimes people want to make larger changes, and this is 
very difficult.  And then we might not need tests like 003_check_guc.pl 
that check the consistency of the sample file.

So here is an initial POC patch.  I have written a script to convert a 
new src/backend/utils/misc/guc_parameters.dat to what would be 
guc_tables.c, but in the patch it's guc_tables_new.c.  The 
guc_parameters.dat in the patch is populated only with a few entries 
that cover most of the different types and variants and possible 
settings, so we can see what it would look like.  Eventually, this would 
require a big conversion.

My next goal would be to make this work so that most of guc_tables.c is 
generated, and nothing else changes beyond that.  But before I do all 
the remaining tedious work, I wanted to check what people think.