Use rtld_putstr() instead of write() for the rtld msg() macro

This removes an unnecessary libc dependency from rtld.
See https://reviews.freebsd.org/D20663 for more details.

libexec/rtld-elf/debug.h

@@ -37,7 +37,7 @@
 #include <sys/cdefs.h>
 
 #include <string.h>
-#include <unistd.h>
+#include "rtld_printf.h"
 
 void debug_printf(const char *, ...) __printflike(1, 2);
 extern int debug;
@@ -57,7 +57,7 @@ extern int debug;
 #define assert(cond)	((cond) ? (void) 0 :		\
     (msg(_MYNAME ": assert failed: " __FILE__ ":"	\
       __XSTRING(__LINE__) "\n"), abort()))
-#define msg(s)		write(STDOUT_FILENO, s, strlen(s))
+#define msg(s)		rtld_putstr(s)
 #define trace()		msg(_MYNAME ": " __XSTRING(__LINE__) "\n")
 
 

libexec/rtld-elf/rtld_printf.h

@@ -31,6 +31,7 @@
 #define RTLD_PRINTF_H 1
 
 #include <sys/cdefs.h>
+#include <stdarg.h>
 #include <unistd.h>
 
 int rtld_snprintf(char *buf, size_t bufsize, const char *fmt, ...)