true, false: perform initialization only when argc == 2

* src/true.c (main): There is no reason to examine argv[0],
call atexit, etc., in the usual case in which we're about to exit.
This has the side effect of making it so that these programs
no longer segfault when subjected to execve abuse.
Before this change, these commands would make "true" segfault:
  printf '%s\n' '#include <unistd.h>' 'int main(int c, char**v)' \
    '{ execve (v[1], 0, 0); }' > k.c && gcc k.c && ./a.out $PWD/true
Now it succeeds.  Reported by Tetsuo Handa and Bart Van Assche
via Ondřej Vašík in http://bugzilla.redhat.com/537684.

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