proc-parse: Add alloc_fhandle/free_fhandle helpers

This removes code duplication. There is no strong
need for free_fhandle but I decided to add it to
be consistent with alloc/free handlers.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>