fix potential out-of-bounds reads

readlink(3) does not nul-terminate the result it sticks
into the supplied buffer. Consequently, the code

  rc = readlink(path, buf, sizeof(buf));

does not necessarily produce a C string.

The code in rc_find_pid() produces some C strings this way
and passes them to strlen() and strcmp(), which can lead
to an out-of-bounds read.

In this case, since the code already takes care to
zero-initialize the buffers before passing them
to readlink(3), only allow sizeof(buf)-1 bytes to
be returned.

(While fixing this issue, I fixed two other locations that
used the same problematic pattern.)

This fixes #270.

src/rc/openrc-run.c

@@ -1152,7 +1152,7 @@ int main(int argc, char **argv)
 	}
 	lnk = xmalloc(4096);
 	memset(lnk, 0, 4096);
-	if (readlink(argv[1], lnk, 4096)) {
+	if (readlink(argv[1], lnk, 4096-1)) {
 		dir = dirname(path);
 		if (strchr(lnk, '/')) {
 			save = xstrdup(dir);