Define and group ppp socket ioctls

Needed for legacy VPN access.

Note that ioctl whitelisting only uses the type and command fields
of the ioctl so only the last two bytes are necessary, thus 0x40047438
and 0x7438 are treated the same.

Bug: 30154346
Change-Id: I45bdc77ab666e05707729a114d933900655ba48b