process: Include missing <signal.h> for raise(3)

Fixes OS X build.
kernel-pfroute: Check for RTM_IFANNOUNCE availability

This message is not available on OS X.
process: Don't use absolute path names for true/false/cat in unit tests

But use the (builtin) shell commands instead, as on OS X true/false are under
/usr/bin.
process: Don't use the shells built-in echo in tests

On OS X, the /bin/sh built-in echo does not support -n.
vici: Return default value for get_int() if message value is empty string

This is the behavior of some strtol() implementations, and it makes sense,
so force it.
thread: Test for pending cancellation requests before select()ing on OS X

This fixes some vici test cases on OS X, where the test thread tries to cancel
the watcher thread during cleanup, but fails as select() does not honor the
pre-issued cancellation request.