Allow PSK/passphrase to be set only when needed

Allow PSK/passphrase to be set only when needed

The new network profile parameter mem_only_psk=1 can be used to specify
that the PSK/passphrase for that network is requested over the control
interface (ctrl_iface or D-Bus) similarly to the EAP network parameter
requests. The PSK/passphrase can then be configured temporarily in a way
that prevents it from getting stored to the configuration file.

For example:

Event:
CTRL-REQ-PSK_PASSPHRASE-0:PSK or passphrase needed for SSID test-wpa2-psk

Response:
CTRL-RSP-PSK_PASSPHRASE-0:"qwertyuiop"

Note: The response value uses the same encoding as the psk network
profile parameter, i.e., passphrase is within double quotation marks.

Signed-off-by: Jouni Malinen <j@w1.fi>