Fix the derivation of the GEOM name from the specified device name by

complementing the existing special case of a not existing /dev prefix
with the recognition of an already existing /dev prefix.

This implicitly solves the following two issues related to working on
GEOM devices /dev/foo/bar (which have the GEOM provider name "foo/bar")
with the expected commands like "bsdlabel /dev/foo/bar":

1. the error "Geom not found" when trying to write or edit the BSD
   label (because previously the incorrect GEOM name "bar" instead of
   "foo/bar" was derived from "/dev/foo/bar").

2. the multiple times reported "magically introduced" partition offset
   of 63 blocks and the resulting errors like "partition extends past
   end of unit" and "partition c doesn't start at 0!".

   This implicitly resulted because bsdlabel(8) determines the "MBR
   offset" via GEOM and (intentionally) silently falls back to an offset
   of 0 if it could not be queried (which is the case if the name was
   incorrectly derived).

   Usually (at least on PCs) the offset for the first slice is 63 blocks
   and bsdlabel(8) automatically subtracts them from the absolute
   offsets in the read on-disk BSD label, resulting in the display of an
   effective offset of 0. If the GEOM query fails, the assumed offset of
   0 is subtracted and an incorrect effective offset of 63 is displayed
   and tried to be worked upon.

Reviewed by: pjd
MFC after: 1 week

sbin/bsdlabel/bsdlabel.c

@@ -222,6 +222,9 @@ main(int argc, char *argv[])
 	} else if (argv[0][0] != '/') {
 		dkname = argv[0];
 		asprintf(&specname, "%s%s", _PATH_DEV, argv[0]);
+	} else if (strncmp(argv[0], _PATH_DEV, strlen(_PATH_DEV)) == 0) {
+		dkname = argv[0] + strlen(_PATH_DEV);
+		specname = argv[0];
 	} else {
 		dkname = strrchr(argv[0], '/');
 		dkname++;