Well it's definitly a permissions issue. I ran into a similar issue once before and it turned out I had "noexec,nosuid" on my /tmp partition in my /etc/fstab. what I had to do was temporarly remove those options from the /etc/fstab and run the make installworld and then add those options back when finished.
You might also want to as your questions on bsdforums.org which is a great forum for questions on the BSDs.
Let me know if this works or if you need more help
Here are my thoughts. A few months ago I had the issue that I pasted below and it turned out I had ran the following command a few days before I started having trouble with make installword. Does your /sbin/init have the immutable flag set?
Try "ls -lo /bin/stuQDdza" To find out.
(Command that caused problems for me)
chflags schg /sbin/*
ie: in your case it might've been "chflags schg /bin/*"
(Error message that i had when I tried to run make installworld afterwards).
install: rename: /sbin/init to /sbin/init.bak: Operation not permitted
*** Error code 71
Stop in /usr/src/sbin/init.
(Command that actually fixed the issue for me)
chflags noschg /sbin/*
ie: This might work in your case "chflags noschg /bin/*
After your make installworld you can then set it back to "chflags schg /bin/*" once again if this is actually what you had set and then disable it once again when you do another make installworld.
Here's what all this stuff does just to give you a little background. The commands below are used for file system security:
schg - set the system immutable flag (superuser only)
The system-level immutable flag can only be set or removed by root. Files with this flag set cannot be changed in any way: not edited, not moved and not replaced. Basically, the kernel will prevent all attempts to touch this file. This flag cannot be removed when the system is running at securelevel 1 or greater.
The superuser-settable "schg" flags can be set at any time by issuing "noschg" instead of "schg", but may only be cleared when the system is running at security level 0 or -1 (insecure or permanently insecure mode, respectively). The securelevel is normally set to 0, for example, when running in single-user mode.
Hope this helps and I wasn't too confusing or long-winded. Let me know if I could help any further.