fetchpatch: add addPrefixes argument

Sometimes patches start without a leading prefix. We default to strip
one prefix or path component from patches (-p1) in the patchPhase in
stdenv.

As all patches should therefore be in this format, fetchpatch should
have an option to normalize patch paths. This commit introduces a new
argument to fetchpatch called addPrefixes that adds one patch prefix to
the old and new paths in a patch before putting it into the store.