linux: Yet more malloc() avoidance for backtrace()

Turns out, there's an initializer at the top of backtrace() that (on
some arches) calls dlopen().  dlopen(), unsurprisingly, calls malloc().
So, call backtrace() early in signal handler setup so we can later
safely call it from the signal handler itself.

os/osinit.c

@@ -59,6 +59,10 @@ SOFTWARE.
 #ifdef HAVE_DLFCN_H
 # include <dlfcn.h>
 #endif
+#ifdef HAVE_BACKTRACE
+#include <execinfo.h>
+#endif
+
 
 #include "dixstruct.h"
 
@@ -192,6 +196,16 @@ OsInit(void)
 		       siglist[i], strerror(errno));
 	    }
 	}
+#ifdef HAVE_BACKTRACE
+	/*
+	 * initialize the backtracer, since the ctor calls dlopen(), which
+	 * calls malloc(), which isn't signal-safe.
+	 */
+	do {
+	    void *array;
+	    backtrace(&array, 1);
+	} while (0);
+#endif
 
 #ifdef RTLD_DI_SETSIGNAL
 	/* Tell runtime linker to send a signal we can catch instead of SIGKILL