minijail: Allow skipping setting securebits when restricting caps

This change allows the user to optionally skip setting a subset of the
securebits that are automatically set when restricting caps.

Bug: 63069223
Test: $ gcc -static -xc -o securebits - << EOF
      #include <stdio.h>
      #include <sys/prctl.h>

      int main()
      {
              printf("%x\n", prctl(PR_GET_SECUREBITS));
      }
      EOF
      $ sudo ./minijail0 -c 1fffffffff --ambient ./securebits
      2f
      $ sudo ./minijail0 -c 1fffffffff --ambient -B 2f ./securebits
      0

Change-Id: Ie247302bbbb35f04caa2066541a8c175f6c94976
5 files changed