vlatai: fix detection of some lujvo as slinku'i

Some lujvo (e.g. {cmakla}) were being detected as slinku'i. This is due to trying to positively match against the slinkuhi rule in vlatai, which is never done elsewhere in the grammar -- it's only used a negative lookahead.

Simply adding !lujvo to slinkuhi doesn't work, because it creates an infinite recursion. So I made another slinkuhi rule for vlatai to use, that only does the !lujvo check once.