libkworkspace: tuning glibc allocator
Some applications like kwin suffer from heap fragmentation
when using the raster graphicssystem. This patch provides a function
that forces glibc to shrink the heap more often.
Now 5*pagesize(=20kB on a typical x86 system) of unused space at the
end of the heap are enough to force free()
to release this space. The default threshold is 128kB which is too much
for an average kworkspace application.
(cherry picked from commit d8949d77b5bc33d540dc7f791cee3691ee9dbb85)