Use pax(1) to build the file system for bsd.rd.

ok pedro@ (for amd64)
ok patrick@ (for armv7)
arm: fix setjmp/longjump by using a different approach

The setjmp regression tests found an issue which I have tried
to fix.  Apparently our current implementation is not 100%
correct, although I have not found the actual failure.

This change is inspired by FreeBSD's code.  Previously we
had a pointer to the jmp_buf and incremented it with every
command we did.  FreeBSD stores the base pointer instead
and accesses the buffer relative to that pointer.

Using this way, setjmp regression tests seem happy now.

"seems mostly reasonable" drahn@
arm: update setjmp to use defines instead of magic values

Update the arm setjmp header with defines from FreeBSD.
Those correspond exactly to how we're doing it now.

Replace magic values in setjmp with those defines.
arm: update PSR bits and modes

ARM has two new modes, hypervisor and monitor, used to
support virtualization.
arm: fix ampintc to actually support more than 32 interrupts
arm: init platform after FDT, so we can use that information
arm: improve agtimer for fdt and kvm support

 * If we're running as virtual machine, use the virtual timer,
   otherwise keep running with the physical.
   Note: We do not yet know how to differentiate both.
 * Use FDT to read and register all the interrupts pins.
 * Do not accept IRQ if istatus is not set.
arm: Implement a simple fdt bus.

Convert virtbus to use it.
Adjusts section 1 of release(8) to make it correct with Bitrig reality.
Initial bits for a complete rework over release(8).
Xr git(1) instead of cvs(1), while here, manually update .Dd, since
nothing will trigger $Mdocdate$ updating, it is a cvs keyword.
Introduces sources(7), a manual page to summarize information on Bitrig
system's source management code and kernel/userland building. This is
one more step towards a rework in the field of online information
resouces, specific for Bitrig's sources management, build and release
procedures.
Adding a few Xr to sources(7) from afterboot(8) and config(8)