Add Google-style docstrings and fix bugs in core modules

[zaihuaji]

• pg_dbi.py: Added docstrings to all 60+ methods; fixed pgbatch uninitialized dict, get_specialist wrong cache key, validate_decs_group wrong variable name
• pg_util.py: Added docstrings to all 54 methods; fixed get_wday NameError, fmtdatetime shadowed built-in str, addyearmonth overwritten params, duplicate adddatetime definition, addtime wrong assignment target, joinarray bare int in for loop, asearch float division index, endtime attribute access typo, and two == None identity checks
• pg_file.py: Added docstrings to all 80+ methods; fixed remote_copy_local dead code after return, record_delete_directory missing re.match arg, compare_md5sum wrong files arg, convert_files op.exist() typo, make_one_backup_directory undefined ret, check_local_executable wrong os.W_OK permission flag
• pg_sig.py: Added docstrings to all methods; fixed os.sleep/sys.sleep ->
  time.sleep, extra self arg in check_process call, get_pbs_info wrong ckeys index, check_background dict mutation during iteration, get_wait_time undefined unit variable fallback