string-table: Introduce DECLARE_STRING_TABLE_LOOKUP() and friends

Let's introduce these for the following two reasons:
- Using them makes sure the parameter names match between declaration
  and definition (if both use the corresponding macros).
- They make sure developers can't forget the _const_ and _pure_
  attributes for the declarations.

This commit also includes some include sorting fixes

src/basic/rlimit-util.h

@@ -7,8 +7,7 @@
 
 #define _RLIMIT_MAX RLIMIT_NLIMITS
 
-const char* rlimit_to_string(int i) _const_;
-int rlimit_from_string(const char *s) _pure_;
+DECLARE_STRING_TABLE_LOOKUP(rlimit, int);
 int rlimit_from_string_harder(const char *s) _pure_;
 void rlimits_list(const char *prefix);