libcutils: fs_config test report aliases

Instead of requiring aliases, let's report when we see
system/<partition>/ all by itself, or in the company of the alias
<partition>/.  Report if we see duplicate entries.  Add checking for
overrides as well.  Report any simple corruptions in internal table
or in the override files.

Test: gTest libcutils_test --gtest_filter=fs_config.*
Bug: 37703469
Change-Id: Ia6a7e8c9bc9f553d0c1c313937b511b2073318a9