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.h

@@ -10,4 +10,8 @@
 #define NULSTR_FOREACH_PAIR(i, j, l)                             \
         for ((i) = (l), (j) = strchr((i), 0)+1; (i) && *(i); (i) = strchr((j), 0)+1, (j) = *(i) ? strchr((i), 0)+1 : (i))
 
-bool nulstr_contains(const char *nulstr, const char *needle);
+const char* nulstr_get(const char *nulstr, const char *needle);
+
+static inline bool nulstr_contains(const char *nulstr, const char *needle) {
+        return nulstr_get(nulstr, needle);
+}