Validate if the executable is dynamically linked

libminijail0 depends on libminijailpreload.so being executed before the
target program, otherwise it will execute the program as root will full
privileges inside the chroot jail (but root can easily escape it).

In order to prevent this, a validation similar to what the Linux kernel
does in load_elf_binary was added to ensure the program was linked
against ld-linux.so.2.