Fix Content-Type in po files to avoid msgfmt warning.

de.po: warning: Charset "iso8859-1" is not a portable encoding name.
                Message conversion to user's charset might not work.

po/de.po

@@ -9,7 +9,7 @@ msgstr ""
 "Last-Translator: Ralf Bchle <ralf@gnu.org>\n"
 "Language-Team:\n"
 "MIME-Version: 1.0\n"
-"Content-Type: text/plain; charset=iso8859-1\n"
+"Content-Type: text/plain; charset=iso-8859-1\n"
 "Content-Transfer-Encoding: 8bit\n"
 
 #: ../arp.c:110 ../arp.c:269

po/fr.po

@@ -9,7 +9,7 @@ msgstr ""
 "Last-Translator: J.M.Vansteene <vanstee@worldnet.fr>\n"
 "Language-Team:\n"
 "MIME-Version: 1.0\n"
-"Content-Type: text/plain; charset=iso8859-1\n"
+"Content-Type: text/plain; charset=iso-8859-1\n"
 "Content-Transfer-Encoding: 8bit\n"
 
 #: ../arp.c:110 ../arp.c:269

po/pt_BR.po

@@ -14,7 +14,7 @@ msgstr ""
 "PO-Revision-Date: 1999-03-01 02:38+0100\n"
 "Last-Translator: Arnaldo Carvalho de Melo <acme@conectiva.com.br>\n"
 "MIME-Version: 1.0\n"
-"Content-Type: text/plain; charset=ISO8859-9\n"
+"Content-Type: text/plain; charset=iso-8859-9\n"
 "Content-Transfer-Encoding: 8bit\n"
 
 #: ../arp.c:110 ../arp.c:269