Fix bug #257.

Emit a warning if the variable that is the subject of a require_complete_switch
scope does not occur in the sub-goal.

compiler/det_report.m:
	As above.

tests/valid/Mmakefile:
tests/valid/bug257b.{m,exp}:
tests/warnings/Mmakefile:
tests/warnings/bug257.{m,exp}:
	Test cases for the above.
Merge branch 'version-13_05-branch'