lib: charset: utility functions for Unicode

utf8_get() - get next UTF-8 code point from buffer
utf8_put() - write UTF-8 code point to buffer
utf8_utf16_strnlen() - length of a utf-8 string after conversion to utf-16
utf8_utf16_strncpy() - copy a utf-8 string to utf-16
utf16_get() - get next UTF-16 code point from buffer
utf16_put() - write UTF-16 code point to buffer
utf16_strnlen() - number of codes points in a utf-16 string
utf16_utf8_strnlen() - length of a utf-16 string after conversion to utf-8
utf16_utf8_strncpy() - copy a utf-16 string to utf-8

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>