ENH: linear solvers: add variable-specific debug flags

Introduces a new optional keyword of label type 'log'
to linear-solver dictionaries to enable variable-specific
debug statements. For example, in fvOptions file:

    solvers
    {
        p
        {
            solver GAMG;
            ...
            log    2;
        }

        U
        {
            ...
            log    0;
        }
    }

The meanings of values of 'log' are:

    log    0;    <!--  no output
    log    1;    <!--  standard output
    log    2;    <!--  debug output
    // values higher than 2 are expected to have no effect

This keyword does not directly affect the operations of various
DebugSwitches and backward compatibility has been ensured in exchange
of code cleanness. The related DebugSwitches are:

    DebugSwitches
    {
        SolverPerformance    0;
        GAMG                 0;
        PCG                  0;
        PBiCG                0;
        smoothSolver         0;
    }