checking file processing functions and let it replace system() that do the same things

[holishing]

• file processing functions: functions in lib/file.c mainly, other related functions also should be considerated.
  • f_cat
  • f_cp
  • f_img
  • f_ln
  • f_exlock
  • f_unlock
  • f_map
  • f_mode
  • f_mv
  • f_new
  • f_open
  • f_rm
  • f_suck
  • directory functions
  • others
• system() functions for processing files: these functions process file via system shell, it should be replaced after checking there's no problem in file processing functions for BBS, or directly use existing functions in standard C libraries.

[holishing]

fd20343 f412834

[IepIweidieng]

For system() calls, they should be replaced with wrapper functions to execv() if no well-tested substitutions are currently available.

Initial works are done in #64 by the introducing of such wrapper functions in lib/proc.c.

Now, only system() calls which use pipes and/or redirections are left and need to be replaced.