lib/charcnv/util_unistr: add next_codepoint_ext() that accepts input charset.

lib/charcnv/util_unistr: add next_codepoint_ext() that accepts input charset.

next_codepoint() takes as string in CH_UNIX encoding and returns the
unicode codepoint of the next (possibly multibyte) character of the
input string.

The new next_codepoint_ext() function adds the encoding of the input
string as a parameter. next_codepoint() now only calls next_codepoint_ext()
with CH_UNIX als src_charset argument.