debugger: fix SIGUSR1 bootstrap race condition

Before this commit, the SIGUSR1 signal handler wasn't installed until
late in the bootstrapping process and we were prone to miss signals
sent by other processes.

This commit installs an early-boot signal handler that merely records
the fact that we received a signal.  Once the debugger infrastructure
is in place, the signal is re-raised, kickstarting the debugger.

Among other things, this means that simple/test-debugger-client is
now _much_ less likely to fail.