liblog: simplify socket_local_client() and always use CLOEXEC

socket_local_client() was copied from libcutils, but we only need a
small subset of its functionality.  We instead create our own version
with just the needed parts.

Importantly, use CLOEXEC in this new function and other places where
it was missing previously.

Test: logging works, liblog-unit-tests
Change-Id: Ifb929227af67bafa13e391eab92358d9f6fe6450