I find very annoying to have to remember the numeric value of secret
flags or have to look them up in the manual every time. Accept the
textual version as well and add support for auto-completion.

 $ nmcli con modify c 802-11-wireless-security.psk-flags not-required

 $ nmcli con modify c 802-11-wireless-security.psk-flags <TAB>
   agent-owned   none          not-required  not-saved