Bump the development sub-version to 6, covering the following major changes

Bump the development sub-version to 6, covering the following major changes
and numerous smaller changes.

    Ffs_balloc deadlock.
    Newreno fixes.
    FreeBSD-SA-05:15.tcp.
    Netgraph module loads.
    Vnode deadlock during tty close.
    Nfs netboot panic.
    Kernel native rpc mount capability.
    Usb/ehci fixes.
    Journaling improvements.
    APIC/SMP fixes.
    TWE/TWA fixes.
    Sync time of day on halt.
    Serializer bug fixed (SMP).
    Token bug fixed (SMP).
    NFS server directory bug fixed.
    (some) Vinum fixes.