Nevin/Warren,
RE: strncpy being related the DIRSIZ
I do not think so, certainly not my memory of programming at the time. Back then, many other languages I was using such as SAIL and some of the Algol family had similarly named functions. I wish he were still with us to ask, but I really think that when Dennis rewrote the V5/V6 "portable C library" into what would become stdio and friends, adding the str*.c family was natural - all the other cool kids had one and it was just a convenient function when we all were writing code for C to have it also.
This is pre "white book" (aka V5 & 6) but I have definite memories of the late Ted Kowalski teaching me what function libraries that were available for C. One of my first programs (after helping Ted with fsck) was rewriting a SAIL based 6502 assembler in C and needing string functions. I have distinct memory of b*tching to Ted about having to write my own string functions.
By the mid-late 70s a number of "ctools"or "c libraries" would appear from UCB, CMU, MIT et al with many of the same basic functions just with slightly different parameters. Go look in the old USENIX tapes, you should see the same stuff getting recreated in many places.
Clem