DragonFlyBSD

This patch enables the re(4) driver to use MSI if the driver supports it.

I have an ASUS M4A87TD motherboard with an integrated RealTek 8168E LAN adapter that supports MSI which I used it for testing.

I used Sephe's bce(4) MSI work as a reference.

Here's the pciconf output with MSI support on:
re0@pci0:1:0:0: class=0x020000 card=0x84321043 chip=0x816810ec rev=0x06 hdr=0x00
cap 01⁴⁰ = powerspec 3 supports D0 D1 D2 D3 current D0
cap 05⁵⁰ = MSI supports 1 message, 64 bit enabled with 1 message
cap 10⁷⁰ = PCI-Express 2 endpoint
cap 11[b0] = MSI-X supports 4 messages in map 0x20
cap 03[d0] = VPD

Here's pciconf output with MSI support turned off:
re0@pci0:1:0:0: class=0x020000 card=0x84321043 chip=0x816810ec rev=0x06 hdr=0x00
cap 01⁴⁰ = powerspec 3 supports D0 D1 D2 D3 current D0
cap 05⁵⁰ = MSI supports 1 message, 64 bit
cap 10⁷⁰ = PCI-Express 2 endpoint
cap 11[b0] = MSI-X supports 4 messages in map 0x20
cap 03[d0] = VPD