I rebooted to find rc.sysinit hanging. I was able to get in thru a mirrored root partition, and to isolate the hang to any reference to 'mount -f'. They all would hang init. Trying to get in with init=/bin/sh would get me in, but had all kinds of problems with 'mount'. Come to find that /etc/ had a slew of files like 'mtab~12345', all with permissions of '-------'. Deleting all these, restored the system's and my sanity. Would suspect some kind of bug in mount that is trying to access these, but an easy workaround would be to do a 'rm -f /etc/mtab*' just before '>mtab' in rc.sysinit.
I might add the events preceding this problem were tarring to a mounted floppy. The floppy filled, and first attempt at umounting failed. Subsequent attempts caused mount to hang and consuming all available CPU. I believe this is where the first mtab~xxxx file originated. There may be room for operator error at this point. What I find as the biggest problem, is that this results in an unbootable system.
Mount is now using mktemp the way it should be.