Hi tech@, was looking through the tree for the providence of the amusing "No really..." message in the installer and saw that the list of invalid users needed updating. Names are in the order found in passwd, with the exception of ftp.
Index: distrib/miniroot/install.sub =================================================================== RCS file: /cvs/src/distrib/miniroot/install.sub,v retrieving revision 1.1000 diff -u -p -r1.1000 install.sub --- distrib/miniroot/install.sub 1 May 2017 14:29:39 -0000 1.1000 +++ distrib/miniroot/install.sub 5 May 2017 18:58:54 -0000 @@ -1945,7 +1945,7 @@ user_setup() { y|yes) _q="No really, what is the lower-case loginname, or 'no'?" continue ;; - root|daemon|operator|bin|sshd|www|nobody|ftp) + root|daemon|operator|bin|sshd|uucp|www|nobody|build|ftp) ;; [a-z]*([a-z0-9_])) ((${#resp} <= 31)) && break