Ignore failures when extracting mtree
Implement the files.txz database, still missing a cool arguments for pkg repo to
activate/deactivate it and still missing pkg filesearch
Enhance plugins management in main().

If a command is not found, when falling-back to plugins, display usage
only if no plugin corresponding to the requested command exist.  If such
a plugin exist and returns an error, do not display usage.

While here remove an inappropriate "not reached" comment on a reached
return.

Fixes this behaviour:
~~~
% pkg Hello World && echo OK
OK
%
~~~
Attempt to display a manual page for plugins

Try do display the pkg-plugin man page.  If it's not available, this
result in:

~~~
% pkg help plugin
No manual entry for pkg-plugin
~~~

which is still better than:

~~~
% pkg help plugin
pkg: 'plugin' is not a valid command.

See 'pkg help' for more information on the commands.
~~~

if such a plugin is available...
Allow pkg action -n (dry-run mode) to succeed without root privs

Fix dry-run logic in pkg_jobs_solve()
Merge pull request #428 from aklv/periodic

Improvements to periodic scripts.
Merge pull request #434 from smortex/warn-invalid-commands

Enhance plugins management in main().
Merge pull request #436 from smortex/plugins-help

Attempt to display a manual page for plugins
add -l flag in usage string, it was already in pkg(8) man page

Conflicts:
	pkg/main.c
add pkg-convert(8) man page and update zsh completion