Goes like "when (<cond>) {<actions>} exit and has same effect as
(and is implemented by) calling built-in pvExit.

This includes the new built-in pvExit, the new exit syntax
for transition clauses, and the seqStop ioc shell command.

This affects safe mode mostly, where we must copy only as many elements
as we got from the pv layer (CA), not the whole array length as
defined in the SNL program.

The previous implementation largely avoids taking locks
but at the cost of being sensitive to thread priorities.
I decided this is too brittle; logical correctness should
not depend on thread priorities or fair scheduling.
A second change closely interwoven with this is to mark
a variable dirty even if its shared buffer is written by a
pvPut (an anonymous PV in safe mode). This removes a special
case and makes anonymous PVs behave more like named ones.

The seqMain.c is a copy of softMain.cpp from EPICS base
with a few minor modifications.

Of course we also provide a default seqMain.c and install it.

Also added a queue with default size to validation tests.