usr.sbin: 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

bhyve: Some fwctl simplifications.

- Collapse IDENT_SEND/IDENT_WAIT states down to a single state.

- Remove unused 'len' argument to op_data callback. The value passed
in (total amount of remaining data to receive) didn't seem very useful
and no op_data implementations used it.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D41286

bhyve: Start moving machine-dependent code into subdirectories

In preparation for an arm64 port, make an easy change which puts some
machine-dependent code in its own directory.

Going forward, code which is only used on one platform should live in a
MD directory. We should strive to layer modules in such a way as to
avoid polluting shared code with lots of ifdefs. For some existing
files this will take some effort.

task_switch.c and fwctl.c are an easy place to start: the former is very
x86-specific, and the latter provides an I/O port interface which can't
be used on anything other than x86. (fwcfg as implemented has the same
problem, but QEMU also supports a MMIO fwcfg interface.) So I propose
that we start by simply making those files conditional.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40501