Refactor var preset parsing in veristat to simplify implementation.
Prepare parsed variable beforehand so that parsing logic is separated
from functionality of calculating offsets and searching fields.
Introduce rvalue struct, storing either int or enum (string value),
will be reused in the next patch, extract parsing rvalue into a
separate function.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>

Implement support for presetting values for array elements in veristat.
For example:
```
sudo ./veristat set_global_vars.bpf.o -G "arr[3] = 1"
```
Arrays of structures and structure of arrays work, but each individual
scalar value has to be set separately: `foo[1].bar[2] = value`.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>

Modify existing veristat tests to verify that array presets are applied
as expected.
Introduce few negative tests as well to check that common error modes
are handled.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>