luci-mod-network: refactor device configuration

Since all netifd device types inherit generic device settings, we can
simplify various aspects of the device configuration ui and drop unused
code while we're at it.

 - Remove setIfActive() helper, superseded by commit
   f3f74bd0 ("luci-base: form.js: consider aliased options in AbstractValue.remove()")

 - Remove most dependency constraints to make all generic device settings
   available for all device types

 - Add MTU value validation to disallow exceeding parent device MTU for
   VLAN interfaces

 - Dynamically update placeholder values when changing base or parent
   device options

 - Undo VLAN network config hack since all options are available now.
   Reverts commit
   3c6b5950

 ("luci-mod-interfaces: simplify configuring MAC address of 802.1q devices")

 - Do not suggest inactive wireless networks as existing device or base
   device choices

 - Disallow specifying names of already existing network devices when
   creating new device settings

 - Fix a number of multicast dependency specifications

 - Drop now unused functions

Signed-off-by: Jo-Philipp Wich <jo@mein.io>