MFC 250418,252166:
Revision 233677 broke certain machines.  Specifically, if the firmware/BIOS
assigned conflicting ranges to BARs then leaving the BARs alone could
result in one device stealing mmio accesses intended to go to a second
device.  Prior to 233677 the PCI bus driver attempted to handle this case
by clearing the BAR to 0 depending on BARs based at 0 not decoding (which
is not guaranteed to be true).  Now when a conflicting BAR is detected the
following steps are taken:

 1) If hw.pci.realloc_bars (a new tunable) is enabled (default is disabled),
    then ignore the current BAR setting from the firmware and attempt to
    allocate a fresh resource range for the BAR.

 2) If 1) failed (or was disabled), disable decoding for the relevant
    BAR type (e.g. disable mem decoding for a memory BAR) and emit a
    warning if booting verbose.