bsdconfig: remove support for ftp, pkg does not support it anymore

bsdconfig: drop support for floppies

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D42083

Merge r248313 from stable/9 sysinstall(8) to head bsdconfig(8):
Add support for installation directly via HTTP.

While we're here, remove the menu-item for Passive FTP (since moving to
ftp(1) and switching FTPMODE to `auto' by default -- see r251613 -- the
single remaining FTP menu-item works for both ftp.f.o and ftp-archive.f.o;
previously each requiring separately active versus passive both work with
the `auto' setting). In scripting you still have mediaSetFTPActive and
mediaSetFTPPassive but the remaining FTP menu-item uses mediaSetFTP which
defaults to `auto' (aforementioned SVN r251613).

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.