add modstat(8)

For example with a rump_server serving FFS:
pooka@watou:~/src/rumprun$ ./rumpremote modstat
NAME             CLASS      SOURCE     REFS  ADDRESS          SIZE     REQUIRES
dk_subr          misc       builtin    0     0                -        -
ffs              vfs        builtin    0     0                -        -
ksem             misc       builtin    0     0                -        -
suser            secmodel   builtin    0     0                -        -
wapbl            vfs        builtin    0     0                -        -