Split out UTF-8 handling functions.
Submit URL-encoded forms in sensible encoding:
* First entry in accept-charset list, if present
* Document encoding, otherwise
We may want to explicitly look for UTF-8, to save converting.
Convert cnv_str_local_enc/cnv_local_enc_str to use iconv (they're now veneers for utf8_[to/from]_enc).
Provide mechanism for looking up local system charset (derived from system alphabet, under RISC OS)
svn path=/import/netsurf/; revision=1647
