feed: Change _USER_AGENT from '+{url}' to '({url})'

In Debian bug 742215 [1], Jakup points out that the old User-Agent is
out-of-spec.  From RFC 2616 [2]:

  User-Agent      = "User-Agent" ":" 1*( product | comment )
  product         = token ["/" product-version]
  product-version = token
  token           = 1*<any CHAR except CTLs or separators>
  separators      = "(" | ")" | "<" | ">" | "@"
                  | "," | ";" | ":" | "\" | <">
                  | "/" | "[" | "]" | "?" | "="
                  | "{" | "}" | SP | HT
  comment         = "(" *( ctext | quoted-pair | comment ) ")"
  ctext           = <any TEXT excluding "(" and ")">

[1]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=742215
[2]: User-Agent: https://tools.ietf.org/html/rfc2616#section-14.43
     product:    https://tools.ietf.org/html/rfc2616#section-3.8
     token, separators, comment, ctext:
                 https://tools.ietf.org/html/rfc2616#section-2.2

Reported-by: Jakub Wilk <jwilk@debian.org>
Signed-off-by: W. Trevor King <wking@tremily.us>