completion: try to complete copy or command

[martinetd]

When using clush -c we should try to complete local files.

When not using -c, if a taget has already been set we can also try to complete a command.
We could do like ssh and try to complete it on any node in the nodeset, but a local completion is probably good enough.

And still complete possible options but only do so if a dash was present, this is similar to what e.g. ls completion does

Fixes #584

[kcgthb]

Thanks for the fix!

I know I'm being a nitpicker here, but two more minor details :)

1. nodeset/group completion works for short options only if there's a space after the option. For instance:

   • clush -w@ log<TAB> completes to clush -w@ login as expected
   • but clush -w@log<TAB> doesn't completes to anything (although -w@login is a valid syntax

2. command completion only works when it's placed before filename completion. But if -c happens first in the command line, then command completion doesn't work anymore. For instance, this doesn't complete the nodeset:

   $ clush -c /etc/slurm/slurm.conf -w @log<TAB>
   

[martinetd]

1. is unrelated to this PR, opened completion: handle short opts without spaces #586
2. fixed

In either case, we allow options to be way too complex... WTH does clush -c file1 -w nodes file2 even work?!

Oh, combining the two patches clush -c<tab> doesn't properly complete file names from the start of the files; I'm tempted to say I don't care enough so just skipping... (as this involves standard bash-completion helpers, while we can lie about $cur I don't know how to re-add the -c prefix at the start of completions... So you won't be able to strip that space down, sorry :P)
Ah, while I'm on weird stuff things like -vwnode also doesn't work. I'll also leave that one as an exercise to the reader...