Use "g" not "f" format in ecpg's PGTYPESnumeric_from_double().

Use "g" not "f" format in ecpg's PGTYPESnumeric_from_double().

The previous coding could overrun the provided buffer size for a very large
input, or lose precision for a very small input.  Adopt the methodology
that's been in use in the equivalent backend code for a long time.

Per private report from Bas van Schaik.  Back-patch to all supported
branches.