various errors resulting from `str.startswith()` matching empty string

[rrotter]

Several places where str.startswith() is used misparse when "" is used as a parameter, because it matches the empty string (""):

• ip addr "" is parsed as ip addr show
• ip "" is parsed as ip addr

What iproute2 does:

% ip a ""
Command "" is unknown, try "ip address help".
% ip ""
Object "" is unknown, try "ip help".

Admittedly this is a degenerate case, and probably isn't too high of a priority.

I started working on these piecemeal, but the patch was getting messy. In most cases making sure that the string isn't "" is sufficient, but I found at least one case where that's still too lenient (ip link s is not allowed by iproute2 in my testing, but maddeningly, ip addr s is!!)

I may submit a patch for this, but probably not soon. I'd probably work on refactoring or unit tests first.

[brona]

I think 28e8dc3 should do it, feel free to have a look / review.