maint: prohibit an operator at end of line

maint: prohibit an operator at end of line

Many coding standards, including GNU's, advocate that when
splitting a line near a binary operator, one should put the
operator at the beginning of the continued line, rather than
at the end of the preceding one.  This is for readability:
such operators are relatively important to readability, and
they are more apparent at the beginning of a line than
at the varying-column end of line,
* cfg.mk (sc_prohibit_operator_at_end_of_line): New rule.
Exempt test.c and head.c.