MFC r274073:
Follow-up to r255036; remove beforeinstall directives from bsdconfig(8)
Makefile's, fixing concurrent installworld (`make -j17 installworld').

Reviewed by: delphij
Thanks to: delphij, emaste

MFC r258355,258360,258400-258401,258406-258407,258418,258430,258439,258589-
258590,258592,258727-258728,258785, with the following descriptions:

Revisions 258400-258401,258406-258407,258590,258727-258728,258785:
+ Add a new `includes' module (see: bsdconfig includes -h) for exploring
the bsdconfig(8) API

Remaining revisions:
258592: Sort all the messages.subr files
258355: Fix typo in a comment in networking/device.subr
258360: Whitespace, style, and other changes in networking/device.subr
258418: Quote interface names for good measure in networking/device.subr
258430: Remove an unused line in networking/device.subr
258439: Minor bugfix to f_die() function in common.subr
258589: Minor bugfix to syscons_ttys module

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

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

Don't prevent the user from hanging their system by changing network
settings while NFS mounts are active; but DO warn them and make the
default action to do nothing. (thanks julian)

Perform some code consolidation and add some additional error
checking/reporting. (similar to SVN revision 251919)

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.

Split IP address validation routines and improve error detection for dotted-
quad notation in IPv6 addresses.

Approved by: adrian (co-mentor) (implicit)

Add IPv6 address-validation routine.

Approved by: adrian (co-mentor) (implicit)

Resurrect and integrate stable/9/sysinstall/help/tcp.hlp

Approved by: adrian (co-mentor) (implicit)

Move major includes into /usr/share/bsdconfig for easy external access.

Reviewed by: adrian (co-mentor)
Approved by: adrian (co-mentor)

Correct double "the the"

Approved by: cperciva
MFC after: 3 days

Import bsdconfig(8) as a replacement for the post-install abilities of
deprecated sysinstall(8). NOTE: WITH_BSDCONFIG is currently required.

Submitted by: Devin Teske (dteske), Ron McDowell <rcm@fuzzwad.org>
Reviewed by: Ron McDowell <rcm@fuzzwad.org>
Approved by: Ed Maste (emaste)