postinstall(8); add -?. expand operation usage

Sprinkle a few references to .tar.xz files (as alternative to .tgz files)

branches: 1.19.4; 1.19.6;
Remove workaround for ancient HTML generation code.

Use Nx.

PR/49428: Travis Paul: Document x option.

branches: 1.16.10;
Remove the ability to specify multiple colon-separated fiel names with
a single "-s" option. Multiple "-s" options must now be used instead.
We have been printing a warning about this since 2008-09-14.

branches: 1.15.6;
s/the the/the/

Explicitly request literal mode after .Xr.

In both postinstall and etcupdate, in modes where the -s argument
refers to tgz files or to a directory in which tgz files have already
been extracted, make it an error for the files that should have come
from etc.tgz to be missing. This is intended to prevent users from
accidentally deleting necessary files when they run "postinstall -s
xetc.tgz fix".

Use the absence of .../etc/mtree/set.etc in the extracted directory
as a test for the error case.

Document that "ss tgz1:tgz2" was merely deprecated, not removed.

Document the change in meaning of "-s foo:bar". It now means a single
file or directory named "foo:bar". If you want the old meaning,
use "-s foo -s bar" instead.

branches: 1.10.2;
Try to make it clear that local changes will be overwritten
by "postinstall fix".

Convert TNF licenses to new 2 clause variant

branches: 1.8.6;
Typo fix.

branches: 1.7.4;
Separate postinstall checks in two groups: enabled and disabled by
default. Only the former checks/fixes are done if no items are given
on the command line. The latter must be requested explicitely.

Intended for "fixes" that are dangerous in some way, because they might
remove files that are still in use, for example.

Make the "sendmail" item disabled by default, it removes sendmail
configuration. Partly addresses PR install/36180.

Proposed on tech-userlevel, review and spelling fixes from lukem@.

* Make postinstall's -s option accept several tgz files, either by
repeating the -s option, or by using a colon-separated list.
* Update postinstall(8) man page with some of the text used in
etcupdate(8)'s description of the -s option.
* Remove an outdated comment about invoking etc/postinstall from
the directory in which the tgz is extracted.
* Rename orig_SRC_DIR to SRC_ARG and make related changes.

Reviewed by lukem and martti.

branches: 1.5.2;
Explain that etcupdate(8) may do the job that postinstall(8) can't
do. (Example: fix master.passwd to include _rwhod and whatnot)

OK'd by lukem@

Xref etcupdate from postinstall, and vice versa.

branches: 1.3.2;
not all items can be fixed automatically

branches: 1.2.2;
Update for move to /usr/sbin.
Add a HISTORY.

Move /etc/postinstall (and the etc.tgz set) to /usr/sbin/postinstall
(and the base.tgz set).