make: replace enum bit-set with struct bit-fields

This makes the code easier to read, especially when setting one of the
flags to false.

No functional change.

make: rename constants for VarFlags

The old prefix was "VAR_" and this prefix is used for several other
constants as well, which made it ambiguous.

make(1): add test that explains how variables are exported

Exporting the variables at the right time and with the correct values is
a subtle issue. The current implementation carefully marks variables as
ready to be exported, then exports them and at the same time tries to
export as few variables as possible, to avoid memory leaks. This test
describes and explains how all this works in detail.

This test also justifies that the call to Var_ReexportVars happens in
the make process itself, not in the child processes, no matter whether
these are created with vfork or (only theoretically) with plain fork.
This has changed in compat.c 1.217, job.c 1.390 and main.c 1.504 from
2020-12-27.