MFC 271965, 272072, 272086: Add QCOW v1 & v2 support

Requested by: gjb

Relnotes:	yes
MFC 272198, 272217:
Add 3 long options (--version, --formats & --schemes) for getting
information about mkimg itself.

Requested by: gjb

Relnotes:	yes
MFC 272384:
Improve performance of mking(1).

Requested by: gjb
MFC r272485: Fill cyclinder, head and sector fields for the EBR,
MBR and PC98 schemes.

Requested by: gjb
MFC r271307:
  Add the ability to set `prefer_source' flag to an IPv6 address.
  It affects the IPv6 source address selection algorithm (RFC 6724)
  and allows override the last rule ("longest matching prefix") for
  choosing among equivalent addresses. The address with `prefer_source'
  will be preferred source address.