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.