jobserver: Ensure the named pipe is deleted on error

jobserver: Ensure the named pipe is deleted on error

Make sure the jobserver named pipe is deleted even if the
jobserver_setup() function calls fatal() and exits early.

* src/main.c (clean_jobserver): Always reset_jobserver().
* src/posixos.c (jobserver_setup): Set job_root up front.
* tests/test_driver.pl: Print out the logfile pathname on error.
For tests with regex matches this might be the only file available.
Don't print the base filename unless one is created.