Fix -Wmissing-variable-declarations compiler warnings.

References:
https://www.illumos.org/issues/3700
Add the Clang specific -Wmissing-variable-declarations to WARNS=6.

This compiler flag enforces that that people either mark variables
static or use an external declarations for the variable, similar to how
-Wmissing-prototypes works for functions.

Due to the fact that Yacc/Lex generate code that cannot trivially be
changed to not warn because of this (lots of yy* variables), add a
NO_WMISSING_VARIABLE_DECLARATIONS that can be used to turn off this
specific compiler warning.

Announced on:	toolchain@
Update chio(1) and ch(4) to support reporting element designators.

This allows mapping a tape drive in a changer (as reported by
'chio status') to a sa(4) driver instance by comparing the
serial numbers.

The designators can be ASCII (which is printed out directly), binary
(which is printed in hex format) or UTF-8, which is printed in either
native UTF-8 format if the terminal can support it, or in %XX notation
for non-ASCII characters.  Thanks to Hiroki Sato <hrs@> for the
explaining UTF-8 printing and example UTF-8 printing code.

chio.h:		Modify the changer_element_status structure to add new
		fields and definitions from the SMC3r16 spec.

		Rename the original CHIOGSTATUS ioctl to OCHIOGTATUS and
		define a new CHIOGSTATUS ioctl.

		Clean up some tab/space issues.

chio.c: 	For the 'status' subcommand, print the designator field
		if it is supplied by a device.

scsi_ch.h:	Add new flags for DVCID and CURDATA to the READ
		ELEMENT STATUS command structure.

		Add a read_element_status_device_id structure
		for the data fields in the new standard. Add new
		unions, dt_or_obsolete and voltage_devid, to hold
		and address data from either SCSI-2 or newer devices.

scsi_ch.c:	Implement support for fetching device IDs with READ
		ELEMENT STATUS data.

		Add new arguments to scsi_read_element_status() to
		allow the user to request the DVCID and CURDATA bits.
		This isn't compiled into libcam (it's only an internal
		kernel interface), so we don't need any special
		handling for the API change.

		If the user issues the new CHIOGSTATUS ioctl, copy all of
		the available element status data out.  If he issues the
		OCHIOGSTATUS ioctl, we don't copy the new fields in the
		structure.

		Fix a bug in chopen() that would result in the peripheral
		never getting unheld if chgetparams() failed.

Sponsored by:	Spectra Logic
Submitted by:	Po-Li Soong
MFC After:	1 week