Add alignment argument to km_map()

Add alignment argument to km_map()

km_map() currently always applies alignment requirement equal to the
size of the mapped region. Most of the time, the natural alignment is
unnecessarily strong and especially on 32-bit systems may contribute to
km_map() failures for regions with size in the order of several hundred
megabytes.

This change adds an extra argument to km_map() which allows the caller
to indicate the desired alignment. The old behaviour can be specified
by passing KM_NATURAL_ALIGNMENT as alignment.

This change only adds the alignment argument, but does not change the
alignment requirement anywhere.