libfdisk: fix NLS support

libfdisk: fix NLS support

The current libfdisk code uses gettext() to translate strings. It
means it follows the default text domain (as set by textdomain(3)
usually in the main program). This is useless for public shared
library.

We have call private bindtextdomain() and use dgettext() with private
domain name to be independent on the main program. For this purpose
include/nls.h supports UL_TEXTDOMAIN_EXPLICIT to use dgettext().

Note that libfdisk will continue to use util-linux.po, rather than
keep the texts in the separate file.

The nls.h has to be included only from fdiskP.h to be sure that
nls.h works as expected for the library.

Signed-off-by: Karel Zak <kzak@redhat.com>