pmb: adjust to distcc 3.3 and wrap it with sshd

Overview

Since Alpine updated to distcc 3.3 last week, pmbootstrap wasn't able to use distcc for cross compilation anymore. It always falled back to running the compiler in QEMU (which works, but is a lot slower). The reason for that is, that distcc requires all compilers that are being used in a whitelist now.

This partially fixes CVE-2004-2687 in distccd, which allowed trivial remote code execution by any process connecting to the distccd server. We only run distccd on localhost, but still this can be used for privilege escalation of sandboxed processes running on the host system (not part of pmbootstrap chroots).

Because the CVE is only partially fixed (see the comment in pmb/chroot/distccd.py for details), we make sure that only the building chroots can talk to the distcc server by running distcc over ssh.

Details

• Completely refactored pmb/chroot/distccd.py to run distcc over ssh
  • Store the running distcc server's arguments as JSON now, not as INI
• Make debugging distcc issues easy:
  • Set DISTCC_BACKOFF_PERIOD=0, so the distcc client will not ignore the server after errors happened (this masks the original error!)
  • New pmbootstrap parameters:
    • --distcc-nofallback: avoids falling back to compiling with QEMU and not throwing an error
    • --ccache-disable: avoid ccache (when the compiler output is cached, distcc does not get used)
  • --verbose prints verbose output of the distcc too
  • New test case, that uses the new pmbootstrap parameters to force compilation through distcc, and shows the output of distcc and distccd in verbose mode on error (as well as the log of sshd)

How to test

This command should work without any error (the test case also runs it):

$ pmbootstrap --ccache-disable --distcc-nofallback \
	build --arch=armhf --force hello-world

Fixes #1619 (closed).