util-lib: unify parsing of nice level values

This adds parse_nice() that parses a nice level and ensures it is in the right
range, via a new nice_is_valid() helper. It then ports over a number of users
to this.

No functional changes.

src/basic/parse-util.c

@@ -29,6 +29,7 @@
 #include "extract-word.h"
 #include "macro.h"
 #include "parse-util.h"
+#include "process-util.h"
 #include "string-util.h"
 
 int parse_boolean(const char *v) {
@@ -551,10 +552,25 @@ int parse_percent_unbounded(const char *p) {
 }
 
 int parse_percent(const char *p) {
-        int v = parse_percent_unbounded(p);
+        int v;
 
+        v = parse_percent_unbounded(p);
         if (v > 100)
                 return -ERANGE;
 
         return v;
 }
+
+int parse_nice(const char *p, int *ret) {
+        int n, r;
+
+        r = safe_atoi(p, &n);
+        if (r < 0)
+                return r;
+
+        if (!nice_is_valid(n))
+                return -ERANGE;
+
+        *ret = n;
+        return 0;
+}