Fix bug in walking .gnu.version_r linked list

When writing the code to teach --replace-needed to modify the
.gnu.version_r section (gh-85), I misunderstood how the ->vn_next
pointers in the Elf_Verneed structs are supposed to be interpreted: I
thought they gave an offset from the beginning of the section, but in
fact they give an offset relative to the current struct. The resulting
bug was very odd: generally, patchelf would complete without signalling
an error, but it would only successfully replace filenames that occurred
as either the first or second entries in the .gnu.version_r section,
while the third or later entries would be left untouched.

This commit fixes the interpretation of the ->vn_next pointers, so that
now --replace-needed should work correctly even on ELF files with more
than two version needed structs.

Thanks to @matthew-brett for finding the bug / providing a test case,
and to @rmcgibbo for helping me diagnose it.

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