init: add "+passcred" for socket to set SO_PASSCRED

In the init scripts for socket, the type can have a suffix of
"+passcred" to request that the socket be bound to report SO_PASSCRED
credentials as part of socket transactions.

Test: gTest logd-unit-tests --gtest_filter=logd.statistics right after boot
      (fails without logd.rc change)
Bug: 37985222
Change-Id: Ie5b50e99fb92fa9bec9a32463a0e6df26a968bfd