use malloc attribute

src/include/string.h

 void * __pure memscan(const void * addr, int c, size_t size) __nonnull;
 char * __pure strstr(const char * s1,const char * s2) __nonnull;
 void * __pure memchr(const void *s, int c, size_t n) __nonnull;
-char * strdup(const char *s) __nonnull;
-char * strndup(const char *s, size_t n) __nonnull;
+char * __malloc strdup(const char *s) __nonnull;
+char * __malloc strndup(const char *s, size_t n) __nonnull;
 
-extern const char * strerror ( int errno );
+extern const char * __pure strerror ( int errno );
 
 #endif /* ETHERBOOT_STRING */