Correct vfork()/exit() to vfork()/_exit().

Correct vfork()/exit() to vfork()/_exit().

Also return "1" rather than "-1".  According to bde: -1 is unrepresentable.
Exit statuses must be >= 0 and <= 255, at least if chars are 8 bits and
shorts are 16 bits.  This seems to only be documented indirectly in exit.2
by referring to wait.2.  WEXITSTATUS() throws away all except the low 8 bits
of the status returned by _exit(), and the kernel actually only stores 8
bits of it (if chars are 8 bits, etc.), so wait() can't return any more bits.

Obtained from: rev 1.4 of contrib/gcc/gcc.c