MINOR: payload: add new sample fetch functions to process distcc protocol

MINOR: payload: add new sample fetch functions to process distcc protocol

When using haproxy in front of distccd, it's possible to provide significant
improvements by only connecting when the preprocessing is completed, and by
selecting different farms depending on the payload size. This patch provides
two new sample fetch functions :

      distcc_param(<token>[,<occ>]) : integer
      distcc_body(<token>[,<occ>]) : binary