fix usage of fcntl(..., F_GETFL, ...)

[tonycoz]

Until about 5 minutes ago (at least in bleadperl), the documentation for fcntl() had included an incorrect example:

use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
    or die "can't fcntl F_GETFL: $!";

This is incorect, fcntl() with the FUNCTION set to F_GETFL returns the value, it doesn't store the result in the SCALAR parameter.

Unfortunately this incorrect usage is reflected in IPC::Cmd, this pull request fixes these calls.

This worked despite the incorrect code because F_SETFL ignores most of the possible flags that F_GETFL returns, eg. F_SETFL won't modify F_RDWR.