This patch changes the unlink operation of the ram fs to defer the
destruction of a file until it is no longer referenced by any VFS handle.
When unlinked, the file no longer appears in the directory. But it can
still be opened and accessed.
With this change, a parent process of a Unix-like subsystem becomes able
to pass the content of an unlinked file to a forked child process. This
mechanism is required when using the 'exec' command in Tcl scripts.
Another use case is the 'tmpfile()' function.
Fixes#3577
