Fix xrefs, sort.

Mention that some platforms may have boot menus, and may support
other boot protocols. Also change some [i386 only] comments to [x86 only].

To get no space between [device:] and [filename],
the mdoc input needs ... Oc Ns Oo ..., not ... Ns Oc Oo ....

Add some examples of machine-specific foo/boot.8 man pages.

Add cross references to new boothowto(9) man page.

Don't workaround ancient macro argument limit with .Xo/.Xc.

Fix preamble to match order set out by mdoc(7). Discussed with wiz.

Improve the description of what the "-c" (RB_USERCONF) flag does.

xref rescue(8)

Update MI boot(8) man page from i386/boot(8)

Synchronise shared text between man8/boot.8, man8/man8.i386/boot.8,
man8/man8.i386/dosboot.8, and man8/man8.i386/pxeboot.8.

* In all:
- First few lines of each man page should be .Dd, .Os, .Dt, .N

Synchronise shared text between man8/boot.8, man8/man8.i386/boot.8,
man8/man8.i386/dosboot.8, and man8/man8.i386/pxeboot.8.

* In all:
- First few lines of each man page should be .Dd, .Os, .Dt, .Nm;
- Use Nx; new sentence, new line; serial comma;
- Update comments about which files to keep in sync.
* In MI boot(8):
- add reference to architecture-specific boot(8);
- add commented-out reference to boot.cfg(5);
- reinstate BUGS heading that was lost when the MI boot.8 was
created from i386 boot.8.
* In i386 boot.8:
- mention -x and -z flags;
- we can boot from media other than floppy, or from the network;
- remove first paragraph in BUGS section now that an MI boot(8)
man page exists.
* In i386 dosboot.8 and pxeboot.8:
- mention -c, -x, and -z flags.

show more ...

Document boot -x

Bump date on previous, emo-style.

Document boot -z

Use Nx; new sentence, new line; serial comma.

First take of MI boot(8) man-page, suggested by agc@.

Okay agc@.