Allow overriding the newline conversion for EBCDIC encodings.

Allow overriding the newline conversion for EBCDIC encodings.

Reported by Mike Fulton <mikefultonpersonal@gmail.com> in
<https://lists.gnu.org/archive/html/bug-gnu-libiconv/2023-04/msg00009.html>.

* include/iconv.h.in (ICONV_SURFACE_NONE,
ICONV_SURFACE_EBCDIC_ZOS_UNIX): New macros.
(ICONV_GET_FROM_SURFACE, ICONV_SET_FROM_SURFACE, ICONV_GET_TO_SURFACE,
ICONV_SET_TO_SURFACE): New macros.
* lib/converters.h (struct conv_struct): Add the fields isurface,
osurface.
(swap_x15_x25): New macro.
* lib/iconv.c (iconv_open, iconv_open_into): Add local variables
from_surface, to_surface.
(ALL_SURFACES): New macro.
(iconvctl): Adjust ICONV_TRIVIALP implementation. Implement the
ICONV_{GET,SET}_{FROM,TO}_SURFACE requests.
* lib/iconv_open1.h: Parse a /ZOS_UNIX surface specifier. Set
from_surface, to_surface.
* lib/iconv_open2.h: Copy the values of from_surface, to_surface into
the conversion descriptor.
* lib/ebcdic*.h (*_mbtowc): Test the isurface. If requested, call
swap_x15_x25 right after fetching an input byte.
(*_wctomb): Test the osurface. If requested, call swap_x15_x25 right
before storing an output byte.
* man/iconvctl.3 (REQUEST VALUES): Document the
ICONV_{GET,SET}_{FROM,TO}_SURFACE requests.
* src/iconv.c (main): If ICONV_EBCDIC_ZOS_UNIX is set, set the from/to
surfaces accordingly.
* man/iconv.1 (ENVIRONMENT): New section.
* tests/check-ebcdic: New file.
* tests/Makefile.in (check): Invoke it.
(SOURCE_FILES): Add it.
* NEWS: Mention the new functionality.