IMP: multiple pending signals

[Saurabh Srivastava]

Although it would make sense to treat all signals as equal, for the
purposes of testing and uniformity you are supposed to handle the signals
with the following priority:

1 - SIGKILL: handled separately anyway!
2 - SIGCHLD
3 - SIGUSR1
4 - SIGUSR2

ie, if you have a SIGCHLD and a SIGUSR2 pending then the SIGCHLD will be
handled before you go and handle SIGUSR2.

[David Marcin]

These signals are defined as:
#define SIGKILL 1
#define SIGUSR1 2
#define SIGUSR2 3
#define SIGCHLD 4

It becomes much more natural to order them differently to comply with
the priority we are supposed to follow. Can we change these #define values?