make jobserver

[bdrewery]

Investigate writing a make-compatible jobserver for sharing tokens down into port builds. It should be relatively easy to implement the server. Then we set the MAKEFLAGS env to contain the jobserver information.

The jobserver client in test/runtest.sh could help here for testing/understanding.

• gmake has 2 modes:
  • --jobserver-auth=fifo:${/path/to/fifo}
  • -jobserver-auth=${fd}
• bmake:
  • -J ${fd}