accept "unwise" characters as valid.  rfc 2396 §2.4.3 says they
aren't valid but they are used all the time in ads.  it probablly
would be good to accept %n grudgingly as this appears
often.  (http://www.computerworld.com/s/article/9138007/Microsoft_No_TCP_IP_patches_for_you_XP)

Notes:
Tue Jan 19 17:06:52 EST 2010 geoff
    should | be escaped too?
    
    if unwise chars appear primarily in ads, isn't
    that a good reason to reject them?


Reference: /n/sources/patch/maybe/webfsunwise
Date: Tue Sep 15 21:11:34 CES 2009
Signed-off-by: quanstro@quanstro.net
Reviewed-by: geoff

--- /sys/src/cmd/webfs/url.c	Tue Sep 15 21:08:26 2009
+++ /sys/src/cmd/webfs/url.c	Tue Sep 15 21:08:23 2009
@@ -111,11 +111,12 @@
  */
 
 /* RE character-class components -- these go in brackets */
+#define UNWISE		"\\[\\]|\\\\^{}`"
 #define PUNCT			"\\-_.!~*'()"
 #define RES			";/?:@&=+$,"
 #define ALNUM		"a-zA-Z0-9"
 #define HEX			"0-9a-fA-F"
-#define UNRES			ALNUM PUNCT
+#define UNRES			ALNUM PUNCT UNWISE
 
 /* RE components; _N => has N parenthesized subexpressions when expanded */
 #define ESCAPED_1			"(%[" HEX "][" HEX "])"