Standardize on MAXPGPATH as the size of a file pathname buffer,

Standardize on MAXPGPATH as the size of a file pathname buffer,
eliminating some wildly inconsistent coding in various parts of the
system.  I set MAXPGPATH = 1024 in config.h.in.  If anyone is really
convinced that there ought to be a configure-time test to set the
value, go right ahead ... but I think it's a waste of time.