use gnulib's proper_name_utf8 function, but *not* proper_name

use gnulib's proper_name_utf8 function, but *not* proper_name

* bootstrap.conf (gnulib_modules): Add propername.
(XGETTEXT_OPTIONS): Add options to tell xgettext about the functions.
* src/cat.c, src/cp.c, src/df.c, src/du.c, src/split.c:
Mark Torbjörn Granlund's name.
* src/ptx.c: Mark François Pinard's name.
Use "TRANSLATORS:" comment marker, rather than "Note to translators:".
* src/system.h: Include propername.h.
(proper_name): Define away.
* src/Makefile.am (cat_LDADD, df_LDADD, du_LDADD, ptx_LDADD, split_LDADD):
Initialize, so we can...
(cat_LDADD, cp_LDADD, df_LDADD, du_LDADD, ptx_LDADD, split_LDADD):
...Use "+=" to append $(LIBICONV) for each program that uses
proper_name_utf8.