basic: delete loadavg.h copy

loadavg.h is an internal header of the Linux source repository, and as
such it is licensed as GPLv2-only, without syscall exception.

We use it only for 4 macros, which are simply doing some math calculations
that cannot thus be subject to copyright.
Reimplement the same calculations in another internal header and delete
loadavg.h from our tree.

src/basic/parse-util.c

@@ -740,13 +740,13 @@ int parse_oom_score_adjust(const char *s, int *ret) {
 int store_loadavg_fixed_point(unsigned long i, unsigned long f, loadavg_t *ret) {
         assert(ret);
 
-        if (i >= (~0UL << FSHIFT))
+        if (i >= (~0UL << PRECISION_BITS))
                 return -ERANGE;
 
-        i = i << FSHIFT;
-        f = DIV_ROUND_UP((f << FSHIFT), 100);
+        i = i << PRECISION_BITS;
+        f = DIV_ROUND_UP((f << PRECISION_BITS), 100);
 
-        if (f >= FIXED_1)
+        if (f >= FIXED_POINT_1_0)
                 return -ERANGE;
 
         *ret = i | f;