Implement --keep-stacktraces=alloc|free|alloc-and-free|alloc-then-free|none

The option --keep-stacktraces controls which stack trace(s) to keep for
malloc'd and/or free'd blocks. This can be used to obtain more information
for 'use after free' errors or to decrease Valgrind memory and/or cpu usage
by recording less information for heap blocks.

This fixes 312913 Dangling pointers error should also report the alloc
stack trace.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13223 a5019735-40e9-0310-863c-91ae7b9d1cf9