Add trailing commas to enum definitions

Since C99, there can be a trailing comma after the last value in an enum 
definition.  A lot of new code has been introducing this style on the 
fly.  I have noticed that some new patches are now taking an 
inconsistent approach to this.  Some add the last comma on the fly if 
they add a new last value, some are trying to preserve the existing 
style in each place, some are even dropping the last comma if there was 
one.  I figured we could nudge this all in a consistent direction if we 
just add the trailing commas everywhere once.  See attached patch; it 
wasn't actually that much.

I omitted a few places where there was a fixed "last" value that will 
always stay last.  I also skipped the header files of libpq and ecpg, in 
case people want to use those with older compilers.  There were also a 
small number of cases where the enum type wasn't used anywhere (but the 
enum values were), which ended up confusing pgindent a bit.