Remove 'imen' global variable from atpic(4).

In pre-SMPng, the global 'imen' was used to track mask state of the
hardware interrupts and was aligned to the masks used by spl*().
When the atpic code was converted to using the x86 interrupt source
abstraction, the global 'imen' was preserved by having each PIC
instance point to an invididual byte in the global 'imen' to hold its
8-bit interrupt mask. The global 'imen' is no longer used for
anything however, so rather than storing pointers in 'struct atpic',
just store the individual 8-bit mask for each PIC as a char.

While here, convert the ATPIC macro to using C99 initializers.

Reviewed by: kib, imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D16827