Document VIS_HTTPSTYLE:

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.

lib/libc/gen/unvis.3

@@ -47,11 +47,14 @@
 .Fn unvis "char *cp" "int c" "int *astate" "int flag"
 .Ft int 
 .Fn strunvis "char *dst" "const char *src"
+.Ft int 
+.Fn strunvisx "char *dst" "const char *src" "int flag"
 .Sh DESCRIPTION
 The
-.Fn unvis
-and
+.Fn unvis ,
 .Fn strunvis
+and
+.Fn strunvisx
 functions
 are used to decode a visual representation of characters, as produced
 by the
@@ -89,6 +92,16 @@ equal to the size of
 decoding).
 .Pp
 The
+.Fn strunvisx
+function does the same as the
+.Fn strunvis
+function, but it allows you to add a flag that specifies the style the string 
+.Ar src
+is encoded with.
+Currently, the only supported flag is
+.Dv VIS_HTTPSTYLE .
+.Pp
+The
 .Fn unvis
 function
 implements a state machine that can be used to decode an arbitrary
@@ -126,10 +139,20 @@ unknown state.  The decoder is placed into the starting state.
 .Pp
 When all bytes in the stream have been processed, call
 .Fn unvis
-one more time with flag set to
+one more time with
+.Ar flag
+set to
 .Dv UNVIS_END
 to extract any remaining character (the character passed in is ignored).
 .Pp
+The
+.Ar flag
+argument is also used to specify the encoding style of the source.
+If set to
+.Dv VIS_HTTPSTYLE ,
+.Fn unvis
+will decode URI strings as specified in RFC 1808.
+.Pp
 The following code fragment illustrates a proper use of
 .Fn unvis .
 .Bd -literal -offset indent

lib/libc/gen/vis.3

@@ -162,7 +162,7 @@ All forms use the backslash character
 to introduce a special
 sequence; two backslashes are used to represent a real backslash.
 These are the visual formats:
-.Bl -tag -width VIS_CSTYLE
+.Bl -tag -width VIS_HTTPSTYLE
 .It (default)
 Use an
 .Ql M
@@ -238,6 +238,13 @@ If
 .Fa nextc
 is an octal digit, the latter representation is used to
 avoid ambiguity.
+.It Dv VIS_HTTPSTYLE
+Use URI encoding as described in RFC 1808.
+The form is
+.Ql %dd
+where
+.Em d
+represents a hexadecimal digit.
 .It Dv VIS_OCTAL
 Use a three digit octal sequence.  The form is
 .Ql \eddd