charconv: Fix the slow-path character conversions

charconv: Fix the slow-path character conversions

This reverts a part of 0189087e257f. That one might have fixed the fast path,
but it broke the slow path. convert_string_internal returns 0/-1 despite the
size_t result type and the misleading comment. If you follow the path to
smb_iconv and for example iconv_copy(), you will see that this routine returns
0 even after it copied something.

How to check: Use German locale, and do an smbclient "allinfo" on a file
modified in march (März in German). Notice the "ä". Before 0189087 this printed
correctly, after 0189087 it cut off the fields after the M for März.

Autobuild-User: Volker Lendecke <vlendec@samba.org>
Autobuild-Date: Tue Mar 29 19:17:41 CEST 2011 on sn-devel-104