Support hex-string input and output for type BYTEA.

Both hex format and the traditional "escape" format are automatically
handled on input.  The output format is selected by the new GUC variable
bytea_output.

As committed, bytea_output defaults to HEX, which is an *incompatible
change*.  We will keep it this way for awhile for testing purposes, but
should consider whether to switch to the more backwards-compatible
default of ESCAPE before 8.5 is released.

Peter Eisentraut