fix jl_load_dynamic_library with NULL argument

This was supposed to return a handle for libjulia, but on unix
systems was returning a handle for the main executable.

src/dlload.c

@@ -139,7 +139,10 @@ static void *jl_load_dynamic_library_(const char *modname, unsigned flags, int t
             jl_error("could not load base module");
         }
 #else
-        handle = dlopen(NULL, RTLD_NOW);
+        Dl_info info;
+        if (!dladdr(&jl_load_dynamic_library, &info) || !info.dli_fname)
+            jl_error("could not load base module");
+        handle = dlopen(info.dli_fname, RTLD_NOW);
 #endif
         goto done;
     }

src/init.c

@@ -299,8 +299,8 @@ void jl_get_builtins(void);
 
 JL_DLLEXPORT void *jl_dl_handle;
 void *jl_RTLD_DEFAULT_handle;
-#ifdef _OS_WINDOWS_
 JL_DLLEXPORT void *jl_exe_handle;
+#ifdef _OS_WINDOWS_
 void *jl_ntdll_handle;
 void *jl_kernel32_handle;
 void *jl_crtdll_handle;
@@ -541,11 +541,6 @@ void _julia_init(JL_IMAGE_SEARCH rel)
     jl_arr_xtralloc_limit = total_mem / 100;  // Extra allocation limited to 1% of total RAM
     jl_find_stack_bottom();
     jl_dl_handle = jl_load_dynamic_library(NULL, JL_RTLD_DEFAULT);
-#ifdef RTLD_DEFAULT
-    jl_RTLD_DEFAULT_handle = RTLD_DEFAULT;
-#else
-    jl_RTLD_DEFAULT_handle = jl_dl_handle;
-#endif
 #ifdef _OS_WINDOWS_
     jl_ntdll_handle = jl_dlopen("ntdll.dll", 0); // bypass julia's pathchecking for system dlls
     jl_kernel32_handle = jl_dlopen("kernel32.dll", 0);
@@ -564,6 +559,13 @@ void _julia_init(JL_IMAGE_SEARCH rel)
     HMODULE jl_dbghelp = (HMODULE) jl_dlopen("dbghelp.dll", 0);
     if (jl_dbghelp)
         hSymRefreshModuleList = (BOOL (WINAPI*)(HANDLE)) jl_dlsym(jl_dbghelp, "SymRefreshModuleList");
+#else
+    jl_exe_handle = jl_dlopen(NULL, JL_RTLD_NOW);
+#ifdef RTLD_DEFAULT
+    jl_RTLD_DEFAULT_handle = RTLD_DEFAULT;
+#else
+    jl_RTLD_DEFAULT_handle = jl_exe_handle;
+#endif
 #endif
 
 #if defined(JL_USE_INTEL_JITEVENTS)