adbd: cleanups in remount/verity.

1. Don't clear O_CLOEXEC on an fd we're duping to stdin/out/err: dup2
   does that for us if oldfd != newfd, and we know the fd can't be
   stdin/out/err.
2. Use unique_fd.
3. Remove gotos by breaking the remount logic into its own function.

Test: mma
Change-Id: If9929e7f20de445322fd498e1379a2d4e572cb52