Miscellaneous minor clarifications.
Until I can revamp the way dependencies are tracked, include all 3
forms of CONFLICTS to make sure we catch alternates.
In parent_exit() run the code to kill background processes if we're
called with a non-zero exit code. That will trigger it (for example)
where the user answers no to the confirmation prompt and is either
using -D, or the generation of the list of valid distfiles did not
complete.

Now that there are 3 sites to trip it, use a flag to indicate that
it needs to be run to make sure that we don't run it more than once.
Improve the handling of the -F option with packages, especially with
--packages-only:

In update_port() do better at saying what we're launching a child to do

In post_first_pass() make clear that we are fetch'ing, not building,
installing, etc.

In multiport() call check_fetch_only when we're done

In the main body add fetch'ed ports to the CUR_DEPS list so that they
are only fetched once.
For multiple -r, don't add a port to the list of dependents to rebuild if
it was itself specified as a -r port.
If an update fails in a child port the parent needs to source the $IPC_SAVE
file before it bails out to make sure that various lists are updated in the
parent. This is particularly important for the list(s) of work already
completed, and especially to update PM_NEEDS_UPDATE for the message of how
to resume from the point of failure.
Add a feature to allow users to list files that should be preserved
across upgrades.
Delete trailing whitespace
Document the PM_PRESERVE_PORTS feature
Unset preserve_port_files when we're done
For the preserve files feature use the same naming and installation
scheme for the new contents file as is done in update_contents()
Add a verbose message for cleaning WRKDIRPREFIX
Fix call to pkg info
Fix typo and indentation
Fix package name of installed port

While the old test would fail, `pkg info -e $port` would also accept
a port name without version, which would result in a message like:

===>>> Currently installed version: gmake

We have to explicitly assign to $upg_port name and version of the
package.
Merge tag '3.11' into integrate-pkgng
Update to portmaster 3.11

The new feature to preserve files across updates is yet to be
implemented.
Add check for existing port when about to expunge

While here, change the hardcoded "pkg_delete" in a message with the
correct tool name.