Bring the libpq example programs into the 21st century.

doc/src/sgml/libpq.sgml

src/test/examples/Makefile

src/test/examples/testlibpq.c

src/test/examples/testlibpq2.c

src/test/examples/testlibpq2.sql

src/test/examples/testlibpq3.c

src/test/examples/testlibpq3.sql

src/test/examples/testlibpq4.c