systemctl: move strv_skip_first() out of systemctl.c

Make it generic, call it strv_skip() and move it to strv.[ch]

src/basic/strv.c

@@ -733,3 +733,15 @@ char ***strv_free_free(char ***l) {
         free(l);
         return NULL;
 }
+
+char **strv_skip(char **l, size_t n) {
+
+        while (n > 0) {
+                if (strv_isempty(l))
+                        return l;
+
+                l++, n--;
+        }
+
+        return l;
+}

src/basic/strv.h

@@ -156,3 +156,5 @@ static inline bool strv_fnmatch_or_empty(char* const* patterns, const char *s, i
 }
 
 char ***strv_free_free(char ***l);
+
+char **strv_skip(char **l, size_t n);