Do not allow unterminated octal escapes in string literals.

library/lexer.m:
	Fix an old XXX: do not allow unterminated octal escapes in string
	literals; we haven't needed to preserve bug-for-bug compatibility
	with NU-Prolog for a very long time.

tests/hard_coded/special_char.m:
	Terminate octal escapes.

tests/invalid/Mmakefile:
tests/invalid/unterminated_octal_escape.{m,err_exp}:
	Check for an error if an unterminated octal escape is encountered.