fix Issue 11879 - missing default User-Agent in std.net.curl

- use "Phobos-std.net.curl/2.065 (libcurl/7.32.0)" as
  default "User-Agent"

- add method to set a different user agent
Merge pull request #1847 from MartinNowak/fix11879

fix Issue 11879 - missing default User-Agent in std.net.curl