Detect stack overflows

Previously, if the Nix evaluator gets a stack overflow due to a deep
or infinite recursion in the Nix expression, the user gets an
unhelpful message ("Segmentation fault") that doesn't indicate that
the problem is in the user's code rather than Nix itself.  Now it
prints:

  error: stack overflow (possible infinite recursion)

This only works on x86_64-linux and i686-linux.

Fixes #35.

src/libmain/shared.cc

@@ -13,6 +13,7 @@
 #include <sys/time.h>
 #include <sys/stat.h>
 #include <unistd.h>
+#include <signal.h>
 
 #if HAVE_BOEHMGC
 #include <gc/gc.h>
@@ -100,6 +101,9 @@ string getArg(const string & opt,
 }
 
 
+void detectStackOverflow();
+
+
 /* Initialize and reorder arguments, then call the actual argument
    processor. */
 static void initAndRun(int argc, char * * argv)
@@ -131,6 +135,9 @@ static void initAndRun(int argc, char * * argv)
     if (sigaction(SIGCHLD, &act, 0))
         throw SysError("resetting SIGCHLD");
 
+    /* Register a SIGSEGV handler to detect stack overflows. */
+    detectStackOverflow();
+
     /* There is no privacy in the Nix system ;-)  At least not for
        now.  In particular, store objects should be readable by
        everybody. */