basic: nulstr-util: add nulstr_get() returning the matching string

Currently `nulstr_contains` returns a boolean, making it difficult to
identify which of the input strings matches the "needle".

Adding a new `nulstr_get()` function, returning a const pointer to the
matching string, eases this process and allows us to directly re-use the
result of a call to this function without additional processing or
memory allocation.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>

src/basic/nulstr-util.c

@@ -3,15 +3,15 @@
 #include "nulstr-util.h"
 #include "string-util.h"
 
-bool nulstr_contains(const char *nulstr, const char *needle) {
+const char* nulstr_get(const char *nulstr, const char *needle) {
         const char *i;
 
         if (!nulstr)
-                return false;
+                return NULL;
 
         NULSTR_FOREACH(i, nulstr)
                 if (streq(i, needle))
-                        return true;
+                        return i;
 
-        return false;
+        return NULL;
 }