Regenerate the list of bsd.progs.mk supported variables

Prefix with dashes (unordered list) and put one variable on each
line (to avoid future conflicts)

Done via the following one-liner:

sh -c 'for i in $(make -C tests/sys/aio PROG=foo -VPROG_VARS:O); do printf "\t\t- $i\n"; done'

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division