mmu: export function to measure the amount of mmaped area

It will be used for procfs compatibility. Applications may want to know how much
memory is potentially available through mmap mappings (not necessarily allocated).

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
sched: export total sys and user time statistics

This is one of the statistics that shows up in /proc/self/stat under Linux, but
this is generally interesting for applications. Since we don't have kernel mode
and userspace mode, it is very hard to differentiate between "time spent in
userspace" and "kernel time spent on behalf of the process". Therefore, we will
present system time as always 0. If we wanted, we could at least differentiate
clearly osv-specific threads as system time, but there is no need to go through
the trouble now.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
linux: provide a /proc/$$/stat compatibility file

MongoDB wants it. In general, I am providing the information that is easy
to get, and ignoring the ones which are not - with the exception of process
count, that seemed easy enough to implement.

This is the kind of thing Mongo does with it:

2014-04-15T09:54:12.322+0000 [clientcursormon] mem (MB) res:670160 virt:25212
2014-04-15T09:54:12.323+0000 [clientcursormon]  mapped (incl journal view):160
2014-04-15T09:54:12.324+0000 [clientcursormon]  connections:0

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>