libfsdriver: support mmap on FSes with no device

This patch adds (very limited) support for memory-mapping pages on
file systems that are mounted on the special "none" device and that
do not implement PEEK support by themselves.  This includes hgfs,
vbfs, and procfs.

The solution is implemented in libvtreefs, and consists of allocating
pages, filling them with content by calling the file system's READ
functionality, passing the pages to VM, and freeing them again.  A new
VM flag is used to indicate that these pages should be mapped in only
once, and thus not cached beyond their single use.  This prevents
stale data from getting mapped in without the involvement of the file
system, which would be problematic on file systems where file contents
may become outdated at any time.  No VM caching means no sharing and
poor performance, but mmap no longer fails on these file systems.

Compared to a libc-based approach, this patch retains the on-demand
nature of mmap.  Especially tail(1) is known to map in a large file
area only to use a small portion of it.

All file systems now need to be given permission for the SETCACHEPAGE
and CLEARCACHE calls to VM.

A very basic regression test is added to test74.

Change-Id: I17afc4cb97315b515cad1542521b98f293b6b559