use AT_EXECFN, if available, for dynamic linker to identify main program

fallback to argv[0] as before. unlike argv[0], AT_EXECFN was a valid
(but possibly relative) pathname for the new program image at the time
the execve syscall was made.

as a special case, ignore AT_EXECFN if it begins with "/proc/", in
order not to give bogus (and possibly harmful) results when fexecve
was used.

src/ldso/dynlink.c

@@ -1016,7 +1016,11 @@ void *__dynlink(int argc, char **argv)
 		}
 		if (app->tls_size) app->tls_image = (char *)app->base + tls_image;
 		if (interp_off) lib->name = (char *)app->base + interp_off;
-		app->name = argv[0];
+		if ((aux[0] & (1UL<<AT_EXECFN))
+		    && strncmp((char *)aux[AT_EXECFN], "/proc/", 6))
+			app->name = (char *)aux[AT_EXECFN];
+		else
+			app->name = argv[0];
 		app->kernel_mapped = 1;
 		app->dynv = (void *)(app->base + find_dyn(
 			(void *)aux[AT_PHDR], aux[AT_PHNUM], aux[AT_PHENT]));