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 makes various DebugSwitches redundant
or reduces their coverage. The effected DebugSwitches are:

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