Change libdisk and sysinstall to use d_addr_t rather than u_long for disk
addresses. For arch's with 64-bit longs, this is a nop, but for i386 this
allows sysinstall to properly handle disks and filesystems > 1 TB.

Changes from the original patch include:

• Use d_addr_t rather than inventing a blkcnt type based on int64_t.
• Use strtoimax() rather than strtoull() to parse d_addr_t's from config files.
• Use intmax_t casts and %jd rather than %llu to printf d_addr_t values.

Tested on: i386
Tested by: kuriyama
Submitted by: julian
MFC after: 1 month