ELF compressed section support in libelf

[Mark Wielaard <mjw at redhat dot com>]

Hi,

Here is another update of the libelf compressed section support.
We decided to bring back [g]elf[32|64]_getchdr and make it so
that compressed Elf_Data has a new Elf_Type ELF_T_CHDR. This makes
some things a little easier/natural. In particular the xlate functions
will now do the right thing with the Chdr at the start of such section
data. Also added is support for elf_strptr to index into compressed
sections (indexes always should be against the uncompressed data).

There are still two XXXs in the code. One for incorrectly marking
the result of the Shdr update by elf_compress dirty. And one to make
sure elf_newdata does the right thing.

There is still some discussion on the final API. But it seems this is
very close to the final version. elf_compress might get a flag argument
and some arguments might turn into unsigned ints.

Comments on the implementations more than welcome. Comments on the
interface should be directed to the Generic System V Application Binary
Interface mailinglist generic-abi(a)googlegroups.com
https://groups.google.com/forum/#!forum/generic-abi

Thanks,

Mark