Fix ustring errors

* mw.ustring.sub( '', 1 ) errors in LuaStandalone
* Default value for ustring.maxStringLength and ustring.maxPatternLength
  should be infinity, not nil
* mw.ustring.find() returns one value instead of two in "plain" mode.

Change-Id: I5e65c4ec3a05f0e6930ce7ab7fd4ac72bea95e7f