This creates a new mode of the foreach command which allows precise
iteration even over empty elements. This mode may be safely extended
with more keyword arguments in the future. The cost now is possibly
breaking scripts that iterate over a list of items beginning with 'IN',
but there is no other way to extend the syntax in a readable way.
This centralizes construction of the error message for an unclosed
logical block (if, foreach, etc.). We record the line at which each
block is opened so it can be reported in the error message.
When a function blocker decides to remove itself we previously removed
it at every return point from the C++ scope in which its removal is
needed. This teaches function blockers to transfer ownership of
themselves from cmMakefile to an automatic variable for deletion on
return. Since this removes blockers before they replay their commands,
we no longer need to avoid running blockers on their own commands.