Resend macros draft

[VictorDenisov]

@dl1jbe @zcsahok I'm working on a fix for #332.
The work is in progress, but I thought maybe you could take an early look and say if you approve the overall direction.
Basically I want to make sendbuf accept qso_values_t type. It's going to use those values to expand macros in the buffer.
Depending on the situation we will be passing current_qso_values or last_qso_values into the sendbuf function.
If callsign field is empty we will be passing last_qso_values otherwise we will be passing current_qso_values.

What do you think?

[zcsahok]

We could maybe drop qsonrstr altogether and use qso_nr of qso_t. qsonrstr seems to be a leftover from the times when even a sprintf was considered a performance hog. This would also save us tracking both values in parallel.

Then macro expansion could either use values from current_qso or from previous_qso pointing to the last QSO in qso_array, as you suggested.

[VictorDenisov]

Closing this PR in favor of #361