kernel - Increase default MAXTSIZ from 128M to 256M

* Increase the default max text size from 128m to 256m.  Note that this
  value can also be overridden in /boot/loader.conf via kern.maxtsiz.

* Currently only chrome compiled w/ full debugging has a text size
  which exceeds 128M.  The normally compiled chrome is hitting 93MB
  though so we might as well up the limit now.