Fixed auto-prepare to not try preparing statements that are not preparable. Bug

src/interfaces/ecpg/preproc/ecpg.addons

src/interfaces/ecpg/preproc/output.c

src/interfaces/ecpg/test/expected/preproc-autoprep.c

src/interfaces/ecpg/test/expected/preproc-autoprep.stderr

src/interfaces/ecpg/test/expected/preproc-autoprep.stdout

src/interfaces/ecpg/test/expected/sql-desc.c

src/interfaces/ecpg/test/expected/sql-execute.c

src/interfaces/ecpg/test/expected/sql-oldexec.c

src/interfaces/ecpg/test/expected/thread-prep.c

src/interfaces/ecpg/test/expected/thread-prep.stdout

src/interfaces/ecpg/test/expected/thread-thread.stdout

...terfaces/ecpg/test/expected/thread-thread_implicit.stdout

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