Improve variant recognition from PGN Event tag

Improve variant recognition from PGN Event tag

Before the Variant tag was invented XBoard extracted the variant name
from the Event tag, and ICS still use this method. The recognition was
too liberal, though, as any occurence of w or wild followed by a number
would be taken as an ICS 'wild' variant. Now we check at least whether
the suspected variant name is not part of a larger word: it must be
preceded by a space, or be at the start of the tag's value string.
  This is also applied to strings that indicate FRC.