[Libc] add postcondition to functions opening file descriptors

headers/header_spec.txt

share/libc/__fc_define_max_open_files.h

share/libc/fcntl.h

share/libc/unistd.h

src/plugins/e-acsl/tests/builtin/oracle_ci/gen_strcat.c

src/plugins/e-acsl/tests/builtin/oracle_ci/gen_strcmp.c

src/plugins/e-acsl/tests/builtin/oracle_ci/gen_strcpy.c

src/plugins/e-acsl/tests/builtin/oracle_ci/gen_strlen.c

src/plugins/e-acsl/tests/builtin/oracle_ci/strcat.res.oracle

src/plugins/e-acsl/tests/builtin/oracle_ci/strcmp.res.oracle

src/plugins/e-acsl/tests/builtin/oracle_ci/strcpy.res.oracle

src/plugins/e-acsl/tests/builtin/oracle_ci/strlen.res.oracle

src/plugins/e-acsl/tests/format/oracle_ci/fprintf.res.oracle

src/plugins/e-acsl/tests/format/oracle_ci/gen_fprintf.c

src/plugins/e-acsl/tests/format/oracle_ci/gen_printf.c

src/plugins/e-acsl/tests/format/oracle_ci/printf.res.oracle

src/plugins/variadic/tests/erroneous/oracle/exec.res.oracle

src/plugins/variadic/tests/known/oracle/exec.res.oracle

...dic/tests/known/oracle/exec_failed_requirement.res.oracle

src/plugins/variadic/tests/known/oracle/fcntl.res.oracle