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 #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.

This is a fixup for commit 3faffc3c59c804db5e1b16fb0b3bf97f07147785.
Merge pull request #1853 from klickverbot/cycle-qualified-constructor

Fix Cycle!(T[N]).opSlice return type qualifier.
Clean up std.getopt docs

After browsing the docs using ddox it is obvious that the standard of "one line short summary and the following lines for in depth descriptions" is not followed in phobos. This commit is mostly to see if such a fixes are wanted/accepted at all.
Merge pull request #1855 from jcd/std-getopt-docclean

Clean up std.getopt docs