race condition with printer setup

[Antoine Martin]

2019-03-05 10:49:01,455 xpra is ready.
^C
2019-03-05 10:49:07,264 got signal SIGINT
2019-03-05 10:49:07,265 exiting
2019-03-05 10:49:11,040 Error: failed to set lpadmin and lpinfo commands
Traceback (most recent call last):
  File "/usr/lib64/python2.7/site-packages/xpra/server/mixins/fileprint_server.py", line 97, in init_printing
    printer_definitions = pycups_printing.validate_setup()
  File "/usr/lib64/python2.7/site-packages/xpra/platform/pycups_printing.py", line 238, in validate_setup
    defs = get_printer_definitions()
  File "/usr/lib64/python2.7/site-packages/xpra/platform/pycups_printing.py", line 206, in get_printer_definitions
    drv = get_lpinfo_drv(x)
  File "/usr/lib64/python2.7/site-packages/xpra/platform/pycups_printing.py", line 127, in get_lpinfo_drv
    cr = getChildReaper()
  File "/usr/lib64/python2.7/site-packages/xpra/child_reaper.py", line 29, in getChildReaper
    singleton = ChildReaper()
  File "/usr/lib64/python2.7/site-packages/xpra/child_reaper.py", line 90, in __init__
    signal.signal(signal.SIGCHLD, self.sigchld)
ValueError: signal only works in main thread

[Antoine Martin]

Fixed in r21970.