Port gdb scripts to python3 / gdb7.7

I tried to use these scripts in an ubuntu 14.04 host (gdb 7.7 compiled
against python 3.3) but there were several errors.

I believe this patch fixes these issues so that the commands
now work (I tested lx-symbols, lx-dmesg, lx-lsmod).

Main issues that needed to be resolved:

  * In python 2 iterators have a "next()" method. In python 3 it is
    __next__() instead (so let's just add both).

  * In older python versions there was an implicit conversion
    in object.__format__() (used when an object is in string.format())
    where it was converting the object to str first and then
    calling str's __format__(). This has now been removed so
    we must explicitly convert to str the objects for which
    we need to keep this behavior.

  * In dmesg.py: in python 3 log_buf is now a "memoryview" object
    which needs to be converted to a string in order to use string
    methods like "splitlines()". Luckily memoryview exists in
    python 2.7.6 as well, so we can convert log_buf to memoryview
    and use the same code in both python 2 and python 3.

This version of the patch has now been tested with gdb 7.7 and
both python 3.4 and python 2.7.6 (I think asking for at least
python 2.7.6 is a reasonable requirement instead of complicating
the code with version checks etc).

Signed-off-by: Pantelis Koukousoulas <pktoss@gmail.com>