Rework current file-system logic

[jounathaen]

Currently, Hermit creates a RAMfs on /, and then (optionally) mounts virtio and uhyve file systems into subdirectories of / (e.g., /root for uhyve).
This approach has the following problems:

• Multiple mounts would be necessary to mount e.g., /etc and /var/log
• it is not possible to mount a single host file in the root directory (e.g., /foo.txt)

Additionally, Uhyve now already provides a file sandbox (thanks to @n0toose) that filters desired files and keeps the host-fs clean.

Solution draft:

• We should default to uhve if available for /, else we use the ramfs.
• We should mount a special filesystem for /proc, /run, /dev and /sys
• We should mount the ramfs for /tmp (and maybe /usr/tmp)
• We could provide a mechanism for specifying other ramfs mounts (overshadowing uhvye)