Fixed incorrect argument handling in SET command if argument is a variable.

src/interfaces/ecpg/ChangeLog

src/interfaces/ecpg/preproc/preproc.y

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

src/interfaces/ecpg/test/expected/sql-show.stderr

src/interfaces/ecpg/test/expected/sql-show.stdout

src/interfaces/ecpg/test/sql/show.pgc