Number 7.

This patch contains four cleanup changes whose aim
is better code self-documentation (the best way to
document IMHO).  They are sent together because they
overlap.

1. Rename the variable "sys_idle" to 'original_pm_idle'.
This is where we store the value that we find in pm_idle before
we substitute the address of our own apm_cpu_idle() function.
In principle we have no idea whose address this is, so
the variable name shouldn't imply that we know that this is
the address of a system idle function; it should simply
indicate that it is the original value of pm_idle.

2. Variable "apm_is_idle" is renamed 'apm_idle_done'.
This flag indicates when apm_do_idle() has been called.
It is a premise of apm_cpu_idle()'s operation that it is
not known whether the apm_do_idle() function really idles
the CPU.  The name of the flag should not lead one to
believe otherwise.

3. Variable "t1" is renamed 'bucket'.  The variable is not
a time but a countdown ("bucket"), so the variable name
should not lead one to believe it is some sort of time
value.

4. A default: case is added to the switch in order to
remind the reader that there is a third possible return
value from apm_do_idle().