The kernel does not log floppy media errors anymore.

In fdcontrol, do always open the file descriptor in read-only mode so
it can operate on read-only media, as there is no longer a separate
control device to operate on.

fdcontrol/fdcontrol.c:
	- Add const constraint to an intermediate value
	  which is not supposed to be changed elsewhere.
fdread/fdread.c:
	- Use _devname in favor of devname to avoid name
	  conflicit.
	- -1 is less than any positive number so in order
	  to get the block to function, we should get the
	  block a little earlier.
	- Cast to remove signed when we are sure that a
	  return value is positive, or is compared with
	  an positive number (tracknumber of a floppy
	  disk is not likely to have UINT_MAX/2 anyway)
fdread/fdutil.c:
	- Use more specific initializer
fdwrite/fdwrite.c:
	- Use static on format_track since it's not
	  referenced in other places.
	- Use const char* to represent string constant.

Bump WARNS accordingly.

Approved by:	das (mentor)

avoid blasting the syslog with error messages from bad floppies.  Both
tools have their own error reporting anyway (which could easily be
cluttered by the syslog output on your terminal).

tools in usr.sbin/fd*.

Use _PATH_* where where possible.