MFC 261524,261526,261527:
- Properly set the alignment flags when allocating the initial range for a
  BAR.  This only really matters when pci_do_realloc_bars is enabled and
  the initial allocation of a specific range fails.
- Simplify pci_reserve_map() by calling resource_list_reserve() to allocate
  the resource after creating a resource list entry rather than reimplementing
  it by hand.
- Add two tunables to ignore certain firmware-assigned resources.  These
  are mostly useful for debugging.
  - hw.pci.clear_bars ignores all firmware-assigned ranges for BARs when
    set.
  - hw.pci.clear_pcib ignores all firmware-assigned ranges for PCI-PCI
    bridge I/O windows when set.