spectool: implement support for SourceScript
    
    This commit adds support for automatically running an external script
    for generating Source files (for example, if the upstream sources
    need to be cleaned of unwanted files and repackaged).
    
    This functionality is triggered if any specified Source file is a
    local file, or when downloading all sources. In this case, the spec
    file contents are parsed and if a line matches the following format
    
        # SourceScript: path-to-script.sh
    
    The script is executed, with the package version supplied as an
    argument:
    
        ./path-to-script.sh $VERSION
    
    In environments where running user-supplied code is not desirable,
    the "--safe" CLI parameter can be used to turn off executing detected
    SourceScript files.