New evhttp_uri(encode|decode) functions to handle + and NUL characters right

New evhttp_uri(encode|decode) functions to handle + and NUL characters right

The old evhttp_decode_uri() function would act as tough it was doing
an (illegal, undefined) decode operation on a whole URL at once, and
treat + characters following a ? as different from + characters
preceding one.  But that's not useful: If you are decoding a URI
before splitting off query parameters, you are begging to fail as soon
as somebody gives you a value with an encoded & in it.

The new evhttp_uridecode() function takes an argument that says
whether to decode + signs.  Both uridecode and uriencode also now
support encoding or decoding to strings with internal 0-valued
characters.