musl: reenable utmp & wtmp paths.

utmp and wtmp file paths defined by musl point to /dev/null.
This patch restores their normal paths.

Notes:
- musl does not have login() and logout() functions that write utmp and wtmp.
- musl has pututline() and related functions, but they are stubs.
- for a program to use utmp/wtmp, it needs to write those files directly.
- Busybox and Dropbear implement direct write to wtmp.
- Busybox implements direct write to utmp too. (Dropbear has a bug.)
- there might be concerns about the security and privacy:
  - OpenWrt is not usually an OS where many persons log into.
  - If an intruder gains access to wtmp/utmp, it probably already has access
    to the rest of the system too.

Signed-off-by: Marius Dinu <m95d+git@psihoexpert.ro>

toolchain/musl/patches/902-utmp-wtmp-paths.patch

@@ -0,0 +1,14 @@
+--- a/include/paths.h
++++ b/include/paths.h
+@@ -18,9 +18,9 @@
+ #define	_PATH_SHADOW	"/etc/shadow"
+ #define	_PATH_SHELLS	"/etc/shells"
+ #define	_PATH_TTY	"/dev/tty"
+-#define _PATH_UTMP	"/dev/null/utmp"
++#define _PATH_UTMP	"/var/run/utmp"
+ #define	_PATH_VI	"/usr/bin/vi"
+-#define _PATH_WTMP	"/dev/null/wtmp"
++#define _PATH_WTMP	"/var/log/wtmp"
+
+ #define	_PATH_DEV	"/dev/"
+ #define	_PATH_TMP	"/tmp/"