introduce ELFShim#rpath, refactor keg_relocate to use the same.

refines PatchELF #runpath, #rpath #soname #interpreter
to return nil.
let Brew hard exits on PatchELF::PatchError.

Co-authored-by: Shaun Jackman <sjackman@gmail.com>
Co-authored-by: Mike McQuaid <mike@mikemcquaid.com>