Completion for :Spawn different from the one for :!

[shym]

I ran into two issues with :Spawn’s completion.

1. If the cursor is at '*' in :Spawn * file, completion proposes (correctly) names for executables. But in :Spawn a* file, it suggests completions of filenames beginning with 'a' instead.
2. The characters that should be escaped (I checked with spaces and semi-colons) in filenames don’t get escaped.

[tpope]

Escaping is easy enough. Positional parameters are really hard to get right. Hopefully my fix doesn't break something else.