Provide a value for Character Encoding for stdlibc (iconv)

[malaterre]

On Debian system, dcmtk is compiled with:

DCMTK_ENABLE_CHARSET_CONVERSION:STRING=stdlibc (iconv)

Which in turn reveal that:

$ dcm2xml --version [...] Character encoding: system default (unknown)

Update implementation detail to retrieve the character encoding using the nl_langinfo() POSIX function.