dsynth - Implement 'test' and 'force', conditional /usr/src, etc

dsynth - Implement 'test' and 'force', conditional /usr/src, etc

* Implement the 'test' and 'force' directives.  Implement
  removal of manually specified packages.  If a generic flavor
  or a meta-port is specified, its direct dependency list will
  also be removed.

* If /usr/src exists under the system directory, mount it in the
  chroot (to support USES=kmod modules).

* Wire up UseCCache but leave disabled and require it to be
  disabled for now.

* Print a build summary at the end.