Extract process-management functionality into a new lib/process

Note that procmon does *not* go here, since procmon needs to
integrate with the event loop.

src/test/test_util_process.c

@@ -7,7 +7,7 @@
 
 #include "test/test.h"
 
-#include "common/util_process.h"
+#include "lib/process/waitpid.h"
 
 #include "test/log_test_helpers.h"