descr

The input/output library <stdio.h> shall not be used

Checks the program for a usage of the next functions defined in the file <stdio.h>:
  - fclose
  - clearerr
  - feof
  - ferror
  - fflush
  - fgetpos
  - fopen
  - fread
  - freopen
  - fseek
  - fsetpos
  - ftell
  - fwrite
  - remove
  - rename
  - rewind
  - setbuf
  - setvbuf
  - tmpfile
  - tmpnam
  - fprintf
  - printf
  - sprintf
  - vfprintf
  - vprintf
  - vsprintf
  - fscanf
  - scanf
  - sscanf
  - fgetc
  - fgets
  - fputc
  - fputs
  - getc
  - getchar
  - gets
  - putc
  - putchar
  - puts
  - ungetc
  - perror

The algorithm is pretty straightforward: we check the input edges for every subroutine
in the program callgraph, and if there are some then the subroutine is considered as a used one,
i.e. there is at least one call of the given subroutine.