Existing passwd and group files overwritten by pkgbase upgrade

[erisianash at gmail.com]

After building and installing -CURRENT r347469 using in-tree pkgbase, I
discovered that my /etc/group, /etc/passwd, /etc/master.passwd,
/etc/pwd.db and /etc/spwd.db had been overwritten. 

I encountered this after running 'make packages', installing into a
ZFS Boot Environment using bectl to create and mount a new BE, running
'pkg -r $mounpoint upgrade -y' and 'pkg -r $mountpoint -yg "FreeBSD-*"'
to upgrade the packages and insure that new base packages got installed,
and finally unmounting and activating the BE.

~Ashley Chase