change the way we identify opened files. The same process on the same

change the way we identify opened files. The same process on the same
machine can open the same file several times (yes, yes, it can), which
wasn't supported correctly.

Now each host has a dynar of 1024 descriptor ids (integers). When
opening a file, a process gets (i.e., pops) an available id for that
host, that it releases (i.e., push back) on close.

MSG_file_dump now shows this id.