redirect stdin when using execl

[Guy Harris]

>Two provisos: System V users will want to use fcntl(, F_DUPFD, ) instead of
>dup2()

Although:

	1) S5R3 has "dup2()"

and

	2) 4.[23]BSD has "fcntl(F_DUPFD)"

so if you're willing to rule out older systems you can avoid polluting
your code with "#ifdef"s.