Improve filtering of backtrace noise in the memcheck bucket

Improve filtering of backtrace noise in the memcheck bucket
and update exp files accordingly. This works well for x86
and all testcases pass on my machine.
New file filter_memcheck to do the work.
There is a bit of a ripple here as filter_memcheck requires
command line arguments to be passed in. So all users of
filter_memcheck (direct or indirect) were updated as well.
filter_stderr was simplified as was filter_libc.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12091