Make SHMMAXPGS an unsigned long

This is used to calculate sizes that are then stored in unsigned long
fields. Make this unsigned long so the calculations use this type and
not an int that can lead to an integer overflow with a large PAGE_SIZE.

This allows building this on arm64 with PAGE_SIZE of 16k. Further work
will be needed if a 32-bit architecture tries to use a similar sized
page.

Sponsored by: The FreeBSD Foundation