Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Import media selection/preparation framework (sysinstall inspired). Makes
accessing files from various types of media nice and abstracted away from
the wet-work involved in preparing, validating, and initializing those
types of media. This will be used for the package management system module
and other modules that need access to files and want to allow the user to
decide where those files come from (either in a scripted fashion, prompted
fashion, or any combination thereof).

Heavily inspired by sysinstall and even uses the same reserved words so
that scripts are portable. Coded over months, tested continuously through-
out, and reviewed several times.

Some notes about the changes:
- Move network-setting acquisition/validation routines to media/tcpip.subr
- The options screen from sysinstall has been converted to a dialog menu
- The "UFS" media choice is renamed to "Directory" to reflect how sysinstall
treats the choice and a new [true] "UFS" media choice has been added that
acts on real UFS partitions (such as external disks with disklabels).
- Many more help files have been resurrected from sysinstall (I noticed that
some of the content seems a bit dated; I gave them a once-over but they
could really use an update).
- A total of 10 media choices are presented (via mediaGetType) including:
CD/DVD, FTP, FTP Passive, HTTP Proxy, Directory, NFS, DOS, UFS, Floppy, USB
- Novel struct/device management layer for managing the issue of passing
more information than can comfortably fit in an argument list.