Make "pgoff_t" be a typedef not a #define.

Make "pgoff_t" be a typedef not a #define.

There doesn't seem to be any great reason why this has been a macro
rather than a typedef.  But doing it like that means our buildfarm
typedef tooling doesn't capture the name as a typedef.  That would
result in pgindent glitches, except that we've seemingly kept it
in typedefs.list manually.  That's obviously error-prone, so let's
convert it to a typedef now.

Discussion: https://postgr.es/m/1681301.1765742268@sss.pgh.pa.us