Implement DO CONTINUE action for ECPG WHENEVER statement.

doc/src/sgml/ecpg.sgml

src/interfaces/ecpg/preproc/ecpg.trailer

src/interfaces/ecpg/preproc/output.c

src/interfaces/ecpg/test/ecpg_schedule

...erfaces/ecpg/test/expected/preproc-whenever_do_continue.c

...es/ecpg/test/expected/preproc-whenever_do_continue.stderr

...es/ecpg/test/expected/preproc-whenever_do_continue.stdout

src/interfaces/ecpg/test/preproc/Makefile

src/interfaces/ecpg/test/preproc/whenever_do_continue.pgc