pwd_mkdb(8): Don't copy comments from /etc/master.passwd to /etc/passwd.

The intention of /etc/passwd was to support legacy applications that are
not yet converted to use modern API like getpwent(3). Comments are not
defined in the legacy format, so copying them could break these
applications. Plus, it could leak sensitive information (e.g. encrypted
form of password of an user that was commented out instead of deleted
or disabled).

PR: bin/144652
MFC after: 1 month