Use MAP_ALIGNED flag to mmap on NetBSD

[daurnimator]

MAP_ALIGNED(n):
Request that the allocation be aligned to the given boundary. The parameter n should be the base 2 logarithm of the desired alignment (e.g., to request alignment to 16K, use 14 as the value for n). The alignment must be equal to or greater than the platform's page size as returned by sysconf(3) with the _SC_PAGESIZE request. The following constants are defined for convenience:

• MAP_ALIGNMENT_16MB
• MAP_ALIGNMENT_4GB
• MAP_ALIGNMENT_1TB
• MAP_ALIGNMENT_256TB
• MAP_ALIGNMENT_64PB

We should use this from the page allocator.