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