add a twistd.hostname on startup

[dustin]

For buildslave roots that are on an NFS or otherwise shared filesystem, it can be difficult to tell which node is running the slave represented by twistd.pid.

For cases like this, a simple write to a twistd.hostname file at startup would probably make things easier. Since the file would not grow unboundedly, I don't think this would cause problems for other users.

[Dustin J. Mitchell]

output the slave hostname to twistd.hostname, for easier management

Fixes #1017

Changeset: f69d4bc5db511ca2972f99d451ab51a913c391dc

[Dustin J. Mitchell]

Delay writing twistd.hostname and logging until after fork

This fixes a fork-thread interaction on Mac OS X (Fixes #1909).

Refs #1017.

Changeset: 34ee24847655d1ff30a2ab496a61a5cd8eb7073b