Makefile.usr: allow easier change of repo URL and cleanups

Makefile.usr: allow easier change of repo URL and cleanups

* Introduce variables GITURL_SRC and GITURL_DPORTS to replace GITHOST and
  GITHOST_DPORTS, respectively, allowing easier change the git repo URLs, e.g.,
  changing to use the HTTPs clone from GitHub.

* Introduce variable BRANCH to specify the src branch to be checked out, either
  "master" (the development branch) or "DragonFly_RELEASE_x_y" (for stable
  releases).  Also print the checkout branch in the help.

* Simplify "src-create" and "src-create-shallow" targets to a one-line git clone
  command with the --depth and --branch options.

* Target "src-create-repo" is merged into "src-create", and target
  "src-checkout" is removed as it is no longer needed.

* Protect "release-sys-extract" target with the existence check of
  src-sys.tar.bz2 .

* Other minor updates and cleanups.