swap, amd64 - increase maximum swap space to 1TB x 4
* The radix can overflow a 32 bit integer even if swblk_t fits in 32 bits.
Expand the radix to 64 bits and thus allow the subr_blist code to operate
up to 2 billion blocks (8TB total).
* Shortcut the common single-swap-device case. We do not have to scan
the radix tree to get available space in the single-device case.
* Change maxswzone and maxbcache to longs and add TUNABLE_LONG_FETCH().
* All the TUNEABLE_*_FETCH() calls and kgetenv_*() calls for integers
call kgetenv_quad().
Adjust kgetenv_quad() to accept a suffix for kilobytes, megabytes,
gigabytes, and terrabytes.