VIS_HTTPSTYLE is a new encoding style for use in vis(), strvis() and
strvisx() that escapes characters according to RFC 1808 (URI encoding).
Since decoding of these require different detection of start-points of
escaped characters, VIS_HTTPSTYLE can be given as flag to unvis().
unvis() will then properly decode URIs.
A new function appeared, strunvisx(): strunvisx() behaves similar as
strunvis(), with one exception: It has an additional flag parameter,
which is passed to unvis() to archive the effect I described above.
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