fs_mgr/init: use unique_ptr + CLO_EXEC for setmntent()/fopen()

We ran into an issue with an fd leaking due to missing both CLO_EXEC
and fclose() in related code, so let's make sure we're safe here too.

Test: boot
Change-Id: Ief893c936859815c78fa6d7e06cb88ad34aadbac
5 files changed