Fix processing of pathnames with embedded '\\' characters (0x5c) in CP932

Fix processing of pathnames with embedded '\\' characters (0x5c) in CP932
character set. Because of the allowing of "broken conversions" for people
who have broken iconv libraries we can't rely on the return from convert_string()
to be valid - we must check errno instead. This is ripe for revisiting at
some stage. I prefer adding a bool parameter to all convert_string_XX varients
to specify if we will allow broken conversions or not. With "allow_broken_conversions"
set to false we could then rely on the return from convert_string rather than
checking errno.
Jeremy.