hostapd: add multi-user vendor element interface

hostapd: add multi-user vendor element interface

Currently hostapd exports a method on a VAP ubus object to set vendor
elements. This does however not work in case multiple external programs
intend to set vendor-specific elements using distinct OUIs / subtypes.

Export two new methods, which allow to set vendor-specific elements for
a specific OUI / subtype combination. This way, multiple callers can set
/ delete vendor elements without affecting each other.

Signed-off-by: David Bauer <[email protected]>