split plt into standard and erbi-specific parts
  Makefile now uses ./.dialyzer.plt which includes info for erbis
  specific deps.  ../dialyzer.plt should contain 'standard' libs.
add erbi_temp_db_helpers:exec_cmd (os commands)
  * os:cmd has deficiencies: it waits for the command to finish,
    and it only returns the output -- you don't get exit status.
      Created exec_cmd helper func, which allows getting
    pid or exit code (can't have both, obviously), and capture output
    user-supplied scanner can perform arbitrary output processing
  * add tests for exec_cmd