fix bugs #1636 and #1837 about scoping of literal strings

src/plugins/e-acsl/Makefile.in

src/plugins/e-acsl/doc/Changelog

src/plugins/e-acsl/env.ml

src/plugins/e-acsl/env.mli

src/plugins/e-acsl/gcc_test.sh

src/plugins/e-acsl/literal_strings.ml

src/plugins/e-acsl/literal_strings.mli

src/plugins/e-acsl/tests/bts/bts1837.i

src/plugins/e-acsl/tests/bts/oracle/bts1390.0.res.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1390.1.res.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1398.0.res.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1398.1.res.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1837.0.err.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1837.0.res.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1837.1.err.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1837.1.res.oracle

src/plugins/e-acsl/tests/bts/oracle/gen_bts1390.c

src/plugins/e-acsl/tests/bts/oracle/gen_bts13902.c

src/plugins/e-acsl/tests/bts/oracle/gen_bts1398.c

src/plugins/e-acsl/tests/bts/oracle/gen_bts13982.c