usnic: do not send zero-length modex message

If there are no usnic BTL modules, then just avoid sending any modex
message at all (other BTLs do this; it's safe to do).

The change is smaller than it looks: I added a "if 0 ==..." check at
the top to return immediately if there are no BTL modules.  Then I
removed some now-unnecessary conditionals and un-indented as
appropriate.
usnic: do not error if modex value not found

It is not an error if a peer does not have a usnic modex message.  It
just means that that peer does not have any usnic BTL modules (and
therefore we can't talk to it).

This has been on trunk/master for quite a while; I just noticed today
that this fix was not on the v1.8 branch.
Merge pull request #44 from jsquyres/pr/usnic-fixes

Minor usnic fixes