Defer all filesystem setup to the user via /etc/fstab

Traditionally, Finit has setup _some_ filesystems that are Nice To
Have (tm), using options that do the Right Thing (tm).

On the one hand, this is very convenient for users that don't
necessarily know what a reasonable mode= value is for /tmp etc. (pun
intended).  It also means that mountpoints can be created if they
don't exist.

On the other hand it means that all users are forced to submit to the
choices made by Finit.  Also, different filesystems where spread out
at many different points in `main()`, meaning that the fact that
`/proc` was available at a given hook point did not imply that `/sys`
was, for example.

Instead, defer _all_ filesystem setup to the user by simply calling
`mount -a`.  We then ship an example `fstab` that shows how to
re-create the old behavior.

contrib/fstab.bb

@@ -0,0 +1,15 @@
+# This is a basic /etc/fstab that mounts up all of the virtual filesystems
+# that older versions of Finit used to mount internally. It can serve as a
+# starting point for systems running Busybox's mount implementation. Make
+# sure that CONFIG_FEATURE_MOUNT_HELPERS enabled; as this is required
+# create the /dev/{pts,shm} directories.
+
+devtmpfs	/dev		devtmpfs	defaults		0	0
+mkdir#-p	/dev/pts	helper		none			0	0
+devpts		/dev/pts	devpts		mode=620,ptmxmode=0666	0	0
+mkdir#-p	/dev/shm	helper		none			0	0
+tmpfs		/dev/shm	tmpfs		mode=0777		0	0
+proc		/proc		proc		defaults		0	0
+tmpfs		/tmp		tmpfs		mode=1777,nosuid,nodev	0	0
+tmpfs		/run		tmpfs		mode=0755,nosuid,nodev	0	0
+sysfs		/sys		sysfs		defaults		0	0