sys: Automated cleanup of cdefs and other formatting

Apply the following automated changes to try to eliminate
no-longer-needed sys/cdefs.h includes as well as now-empty
blank lines in a row.

Remove /^#if.*\n#endif.*\n#include\s+<sys/cdefs.h>.*\n/
Remove /\n+#include\s+<sys/cdefs.h>.*\n+#if.*\n#endif.*\n+/
Remove /\n+#if.*\n#endif.*\n+/
Remove /^#if.*\n#endif.*\n/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/types.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/param.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/capsicum.h>/

Sponsored by: Netflix

sys: Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

xdma: Remove unused devclass arguments to DRIVER_MODULE.

Enter the network epoch in the xdma interrupt handler if required
by a peripheral device driver.

Sponsored by: DARPA, AFRL

Tune xDMA interface slightly:
o Move descriptors allocation to DMA engine driver
o Add generic xdma_request() routine
o Add less-generic scatter-gather application based on xdma interface

Typical operation flow in peripheral device driver is:

1. Get xDMA controller
sc->xdma_tx = xdma_ofw_get(sc->dev, "tx");

2. Allocate virtual channel
sc->xchan_tx = xdma_channel_alloc(sc->xdma_tx, caps);

3. Setup transfer status callback
xdma_setup_intr(sc->xchan_tx, my_tx_intr, sc, &sc->ih_tx);

4. Request a transfer(s)
ret = xdma_request(sc->xchan_tx, &req);

5. Free the channel
xdma_channel_free(sc->xdma_tx);

6. Free the controller
xdma_put(sc->xdma_tx);

Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14971

Add xDMA -- the DMA abstraction layer, initial verison.

xDMA is a DMA framework designed to abstract the interaction
between device drivers and DMA engines.

Project wiki: https://wiki.freebsd.org/xdma

Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8807