dsynth - Optimize pty use

dsynth - Optimize pty use

* Have the WORKER process allocate the pty once and use it in all
  phases, instead of allocating and closing a new pty for each phase.

* Make N copies of /bin, /lib, /usr/bin, and /libexec into the base
  directory and mount those in the chroots instead of the system
  directory, essentially unsharing these directories.  Each worker
  gets an independent copy.

  This should reduce contention and cache-line ping-ponging overhead
  on /bin/sh, libc, make, etc, significantly.

* Create the template conditionally whether 'pkg' has been built or
  not.