buildworld - Fix buildworld conflict

* Remove EX_REBOOT from include/sysexits.h, it conflicts with the
  buildworld.

* Replace EX_REBOOT with a local #define'd symbol in
  vkernel64/platform/init.c.
Fix typos (sytem -> system) in a manpage and a message.
kernel/drm: ttm_mem_global_free() is void, so don't return any result.