Implement the _TALLY system variable for the COUNT command

[Irwin1985]

Fixes #2024, or at least the COUNT part of it.

Two things were broken. COUNT without the TO clause didn't even compile: the VO rule in dbcmd.xh expands to "<xvar> := 0", so leaving TO out gives you a naked := and an XS9002. And _TALLY was declared in SystemVariables.prg but nothing in the runtime ever wrote to it, so COUNT TO n gave you n = 2 and _TALLY = 0.

Added three COUNT rules plus a __VfpCount() helper in Commands.prg. The helper just wraps DbEval(), same as the VO rule did, but counts inside and stores the result in _TALLY.

[RobertvanderHulst]

Irwin,
I am considering to add a setting to the runtime, where dbeval() stores the # of records processed in it's last run.
Something similar to the field where the error code for the low level file operations is stored.
That can then be used to assign _tally but is also available for other code.
I cannot assign to _tally directly since that field is inside the vfp assembly.
I will create a local inside the dbeval code and assign the setting after dbeval() finishes.
What do you think of this?

[Irwin1985]

Hi Robert,

Sounds good to me. My helper only counts for COUNT, every other command would need its own. One thing I hit while checking this against VFP 9 is that _TALLY is the records affected, not the ones visited. With 5 records and 1 deleted, RECALL ALL reports 1, but our RECALL goes through DbEval with scope ALL, so a counter in there would say 5. Might need special casing.