restore behavior of -e-acsl-valid which was broken since Jan. 2017 or so

src/plugins/e-acsl/.gitignore

src/plugins/e-acsl/Makefile.in

src/plugins/e-acsl/doc/Changelog

src/plugins/e-acsl/dup_functions.ml

src/plugins/e-acsl/dup_functions.mli

src/plugins/e-acsl/keep_status.ml

src/plugins/e-acsl/keep_status.mli

src/plugins/e-acsl/main.ml

src/plugins/e-acsl/misc.ml

src/plugins/e-acsl/misc.mli

src/plugins/e-acsl/options.ml

src/plugins/e-acsl/options.mli

src/plugins/e-acsl/prepare_ast.ml

src/plugins/e-acsl/prepare_ast.mli

src/plugins/e-acsl/tests/bts/oracle/bts1390.res.oracle

src/plugins/e-acsl/tests/bts/oracle/bts2192.res.oracle

src/plugins/e-acsl/tests/bts/oracle/gen_bts1307.c

src/plugins/e-acsl/tests/bts/oracle/gen_bts1324.c

src/plugins/e-acsl/tests/bts/oracle/gen_bts1326.c

src/plugins/e-acsl/tests/bts/oracle/gen_bts1395.c