Add rint() to /port, remove from qnx/.

configure

configure.in

src/backend/port/qnx4/Makefile

src/backend/port/qnx4/tstrint.c

src/include/c.h

src/interfaces/ecpg/pgtypeslib/Makefile

src/interfaces/ecpg/pgtypeslib/dt.h

src/interfaces/ecpg/pgtypeslib/dt_common.c

src/port/rint.c