On z/OS, set a charset tag on iconv's output file.

For the concept of charset tags as external metadata on z/OS files, see

* src/zos-tag.h: New file.
* src/iconv.c: Include zos-tag.h.
(convert): Add a 'tocode' parameter. On z/OS, turn off auto-conversion
and tag the output file.
(main): Update callers.
* tests/check-ebcdic: On z/OS, make all test files initially untagged.
* tests/check-tag: New file.
* tests/Makefile.in (check): Pass the host_os to check-ebcdic. Invoke
6 files changed