Pavel reported that in case if there a big number
of small vmas present in the dumpee we're reading
/proc/pid/pagemap too frequently.

To speedup this procedue we inroduce pagemap cache.

The interface is:
 - pmc_init/pmc_fini for cache initialization and freeing
 - pmc_get_map to retrieve specific PMEs array for VMA area

v2:
 - Move internal constants to pagemap-cache.c
 - Make PAGEMAP_LEN to accept virtual address/size
 - Don't adjust low bound in caching mode to save a couple of code bytes
Reported-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>