Implement VG_(arena_realloc_shrink) similar to realloc, but can

Implement VG_(arena_realloc_shrink) similar to realloc, but can
only decrease the size of a block, does not change the address,
does not need to alloc another block and copy the memory,
and (if big enough) makes the excess memory available for other
allocations.

VG_(arena_realloc_shrink) is then used for debuginfo storage.c
(replacing an allocation + copy).
Also use it in the dedup pool, to recuperate the unused
memory of the last pool.
This also allows to re-increase the string pool size to the original
3.9.0 value of 64Kb. All this slightly decrease the peak and in use
memory of dinfo.

VG_(arena_realloc_shrink) will also be used to implement (in another patch)
a dedup pool which "numbers" the allocated elements.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14122