strv: multiple cleanups

- turn strv_merge into strv_extend_strv.
   appending strv b to the end of strv a instead of creating a new strv
- strv_append: remove in favor of strv_extend and strv_push.
- strv_remove: write slightly more elegant
- strv_remove_prefix: remove unused function
- strv_overlap: use strv_contains
- strv_printf: STRV_FOREACH handles NULL correctly

src/modules-load/modules-load.c

@@ -64,13 +64,9 @@ static int add_modules(const char *p) {
         if (!k)
                 return log_oom();
 
-        t = strv_merge(arg_proc_cmdline_modules, k);
-        if (!t)
+        if (strv_extend_strv(&arg_proc_cmdline_modules, k) < 0)
                 return log_oom();
 
-        strv_free(arg_proc_cmdline_modules);
-        arg_proc_cmdline_modules = t;
-
         return 0;
 }