strv: introduce strv_sort_uniq()

We often call strv_sort() and strv_uniq(). If a strv is already sorted.
uniquifying can be faster.

Prompted by https://github.com/systemd/systemd/pull/33012#discussion_r1636633627.

src/basic/strv.c

@@ -704,6 +704,21 @@ char** strv_sort(char **l) {
         return l;
 }
 
+char** strv_sort_uniq(char **l) {
+        if (strv_isempty(l))
+                return l;
+
+        char **tail = strv_sort(l), *prev = NULL;
+        STRV_FOREACH(i, l)
+                if (streq_ptr(*i, prev))
+                        free(*i);
+                else
+                        *(tail++) = prev = *i;
+
+        *tail = NULL;
+        return l;
+}
+
 int strv_compare(char * const *a, char * const *b) {
         int r;
 

src/basic/strv.h

@@ -161,6 +161,7 @@ bool strv_overlap(char * const *a, char * const *b) _pure_;
         _STRV_FOREACH_PAIR(x, y, l, UNIQ_T(i, UNIQ))
 
 char** strv_sort(char **l);
+char** strv_sort_uniq(char **l);
 void strv_print_full(char * const *l, const char *prefix);
 static inline void strv_print(char * const *l) {
         strv_print_full(l, NULL);