Fix exec of shell script

[rohgarg]

This fixes issue #676.

[jiajuncao]

Is it possible that the interpreter itself is another script, which has another interpreter (I have not seen such a scenario, nor do I know if it is valid)? In that case, recursion is possible. If it it is true, we can just put the string comparison inside elfType(), since we are doing the magic number stuff anyway, and recursively call itself if the first line is an interpreter.

[gc00]

1. I think that the goal is thatelfType() will work on an ELF binary, and getInterpreterType() will work on a script whose interpreter is an ELF binary. But a sciprt could have an interpreter that's a script. Why not turn this into a recursive function in which getInterpreterType() is the recursive function, which first directly calls the base case (elfType()), and then if that fails, it calls the current body of getInterpeterType()? If you're worried about infinite recursion for some weird buggy script, then you can limit to 10 tries or something.

2. Note that this then allows you to clean up the excerpts of code in src/dmtcp_launch.cpp and src/execwrappers.cpp. Instead of callilng both functions, you can simply call one recurseve function.

3. Do you have a test case where exec of a shell script is failing under DMTCP, but succeeds under direct Linux? I tried to create an example, and failed. If you have a test case, could you add it to the test directory (autotest.py)?

Finally when I tried to produce a test case example, here are the pieces that I tried to build up.
The code corresponds, respectively, to the fiels: tmp.c, tmp2.sh, tmp.sh. Hopefully, this is helpful to you:

int main() {
char *cmd[] = {"/tmp/tmp2.sh", "ls", NULL};
execve(cmd[0], cmd, NULL);
}

#!/tmp/tmp.sh
exec $1

#!/usr/bin/python
import os
import sys
os.system(sys.argv[1])

[gc00]

LGTM.