Stubbed the <stdio.h> feof(), fgetc(), getc(), getchar() functions.
Ensured that __sys_exit() calls exit_group(2) instead of exit(2).
Defined all <string.h> functions from the C11 standard.
Implemented the <string.h> strerror() function.
Implemented the <string.h> strspn() function.
Implemented the <string.h> strrchr() function.
Implemented the <string.h> strpbrk() function.
Implemented the <string.h> strcspn() function.
Implemented the <string.h> strchr() function.