Import compiler warnings and bugfixes from Charles.

This change imports a few warnings and minor fixes from Charles branch
here: https://bitbucket.org/plan9-from-bell-labs/plan9.

The changes included here:


	changeset:   1374:9185dc017be0
	summary:     declare castucom; move a declaration into order;
	             use cast instead of ULL suffix
	changeset:   1353:5fe8380b1818
	summary:     supporting functions:
	             1. castucom to match unlikely mask operation;
  	             2. be sure to snap both sides of pointer subtraction completely;
	             3. add extra operators as side-effect free
	changeset:   1352:90058c092d66
	summary:     1. correct result type for mixed-mode assignment operators
	             2. detect divide by zero (erik);
	             3. detect masks misformed by sign-extension;
	             4. diagnose mixed old/new prototypes

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