Revert "Replaced select() calls with waitpid() in slave.c when waiting for the greeter to die (fixes zombie greeter process hanging MDM slave)"

This reverts commit 0b059f0809f8e4172f7a079e25d5bbe865c0c152.
Use waitpid to "whack" the greeter (workaround for a regression in Ubuntu 13.04), continue to use select() for session/chooser output (prevents session freeze when output is sent to .xsession-errors)