mkfs has been replaced with the newfs utility.
As for the FAT partition:
Pay special attention to the -m mask option. Hint:
/dev/ad1s1 /d msdos rw,-m=777 0 0
I am not sure of what the "user" option did for you in Linux. If you would tell me there probably is something similar. You don't want to dump or check this partition so keep the last two numbers as 0s.
This line will mount it so that every file is readable, writable, and executatable by everyone on the system. Very good security I assure you.