Add the "-t" option to geom(8) utility, to display geoms hierarchy.

Add the "-t" option to geom(8) utility, to display geoms hierarchy.
Sample output:

% geom -t
Geom             Class      Provider
da0              DISK       da0
  da0            PART       da0s1
    da0s1        PART       da0s1a
      ffs.da0s1a VFS
      da0s1a     DEV
    da0s1        DEV
  da0            DEV
da1              DISK       da1
  swap           SWAP
  da1            DEV
cd0              DISK       cd0
  cd0            DEV

Reviewed by:	oshogbo
Approved by:	re (kib)
MFC after:	2 weeks
Relnotes:	yes
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D17151