passes: check if .so file exists before loading

This will suppress all the 'File does not exists' warnings.
passes: simplify and extend searching of .so files

Search for the passes in:
    local subdir (for developing)
    $HOME/.wldbg
    /usr/local/lib/wldbg
    /usr/lib/wldbg
    /lib/wldbg

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
passes: remove comma from arguments

If the passes are separeted by the comma, remove it from the last
argument.
pass: add help pass

This pass just prints information about wldbg and is used for debugging
atm. Later it can contain information about wldbg

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>