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

In the init scripts for socket, the type can have a suffix of
"+cred" to request that the socket be bound to report SO_PASSCRED
credentials on socket transactions.  Here we add socket setopt
to selinux rules.

Test: gTest logd-unit-tests --gtest_filter=logd.statistics right after boot
      (fails without logd.rc change)
Bug: 37985222
Change-Id: I37cdf7eea93c3e8fa52964e765eaf3007e431b1f