Added STRING datatype for Informix compatibility mode. This work is

src/interfaces/ecpg/ChangeLog

src/interfaces/ecpg/compatlib/Makefile

src/interfaces/ecpg/ecpglib/data.c

src/interfaces/ecpg/ecpglib/descriptor.c

src/interfaces/ecpg/ecpglib/execute.c

src/interfaces/ecpg/ecpglib/misc.c

src/interfaces/ecpg/ecpglib/typename.c

src/interfaces/ecpg/include/ecpgtype.h

src/interfaces/ecpg/pgtypeslib/Makefile

src/interfaces/ecpg/preproc/ecpg.header

src/interfaces/ecpg/preproc/ecpg.trailer

src/interfaces/ecpg/preproc/type.c

src/interfaces/ecpg/preproc/variable.c

src/interfaces/ecpg/test/compat_informix/test_informix.pgc

...rfaces/ecpg/test/expected/compat_informix-test_informix.c

...s/ecpg/test/expected/compat_informix-test_informix.stderr

...s/ecpg/test/expected/compat_informix-test_informix.stdout