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

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

next_codepoint_convenience() 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_convenience_ext() function adds the encoding of the input
string as a parameter. next_codepoint_convenience() now only calls
next_codepoint_convenience_ext() with CH_UNIX als src_charset argument.