Resurrect gdb-add-index as a contrib script

Resurrect gdb-add-index as a contrib script

This includes changes made in Fedora's gdb packaging[1], Doug's
robustness patch[2] from before gdb-add-index was dropped, some
corrections, and some more changes Doug accumulated in the
meantime[3].

[1]: http://pkgs.fedoraproject.org/cgit/gdb.git/log/gdb-gdb-add-index-script.patch?id=fe74423b0812bae6d7bb027584e401a2ac37d24d
[2]: https://sourceware.org/ml/gdb-patches/2010-09/msg00130.html
[3]: https://sourceware.org/ml/gdb-patches/2013-11/msg00297.html

It would be a good idea to mention the existance of this script
in (info "(gdb) Index Files"), but I'm boycotting invariant
sections/cover texts because non-free docs are a PITA, so somebody
else would need to do that.

Summary of previous activity:

97924a9 Actual removal
c29c521 Attempted removal (accidentally left gdb-add-index.sh in place)
c2bbed2 Addition