a global static variable.
(class mmap_record): Add `devtype_' member to remember
the device type of the file. Add declaration for methods
`get_device', `alloc_fh' and `free_fh'.
(mmap_record::mmap_record): Initialize `devtype_' correctly.
(mmap_record::alloc_fh): New method.
(mmap_record::free_fh): Ditto.
(munmap): Use new mmap_record::alloc_fh and mmap_record::free_fh
methods to create a correct fhandler.
(msync): Ditto.
(fixup_mmaps_after_fork): Ditto.