add PROJECT=phobos ddoc macro

- This will be used to customize links in dlang.org's std.ddoc.
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 #1852 from klickverbot/cycle-qualified-constructor

Avoid calling a qualified constructor in Cycle.
Merge pull request #1846 from MartinNowak/projectDDOC

add PROJECT=phobos ddoc macro
Merge pull request #1847 from MartinNowak/fix11879

fix Issue 11879 - missing default User-Agent in std.net.curl
Fix Cycle!(T[N]).opSlice return type qualifier.