bit flags ORed together.  Currently the only flags defined are
_PASSWORD_SECUREONLY and _PASSWORD_OMITV7 but this is enough to
cause pw_mkdb() to run pwd_mkdb with the options we want.
With this change we no longer generate the old V7 passwd file when
only the extra fields in master.passwd (or the encrypted password)
have changed.  There are other programs that could probably use
the _PASSWORD_OMITV7 flag; they will be converted at a future date.