udev: add flag to allow disabling blkid probing

This can be useful for users of slow block devices.

For example, the persistent-storage rules are needed for USB floppy
drives be recognized by udisks2, but the extra blkid calls cause
thrashing for 25+ seconds after every disk change.

With this change, a user wishing to avoid the extra blkid invocation(s)
could create /etc/udev/rules.d/55-floppy-noprobe.rules as follows:

    # Don't probe PC floppy drives
    SUBSYSTEM=="block", KERNEL=="fd*", \
        ENV{UDEV_DISABLE_PERSISTENT_STORAGE_BLKID_FLAG}="1"

    # Don't probe USB floppy drives
    SUBSYSTEM=="block", SUBSYSTEMS=="usb", \
        ATTRS{bInterfaceClass}=="08", ATTRS{bInterfaceSubClass}=="04", \
        ENV{UDEV_DISABLE_PERSISTENT_STORAGE_BLKID_FLAG}="1"

I didn't exclude floppies by default in this change, because floppy
devices are also emulated by some BIOSes/hypervisors in some cases, and
I don't know how many systems would fail to boot if /dev/disk/by-uuid/*
became unavailable for 'floppy disks' on those systems.

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