core: add support for naming file descriptors passed using socket activation

This adds support for naming file descriptors passed using socket
activation. The names are passed in a new $LISTEN_FDNAMES= environment
variable, that matches the existign $LISTEN_FDS= one and contains a
colon-separated list of names.

This also adds support for naming fds submitted to the per-service fd
store using FDNAME= in the sd_notify() message.

This also adds a new FileDescriptorName= setting for socket unit files
to set the name for fds created by socket units.

This also adds a new call sd_listen_fds_with_names(), that is similar to
sd_listen_fds(), but also returns the names of the fds.

systemd-activate gained the new --fdname= switch to specify a name for
testing socket activation.

This is based on #1247 by Maciej Wereski.

Fixes #1247.

src/basic/strv.c

@@ -277,8 +277,8 @@ char **strv_split_newlines(const char *s) {
 }
 
 int strv_split_extract(char ***t, const char *s, const char *separators, ExtractFlags flags) {
-        size_t n = 0, allocated = 0;
         _cleanup_strv_free_ char **l = NULL;
+        size_t n = 0, allocated = 0;
         int r;
 
         assert(t);
@@ -302,13 +302,16 @@ int strv_split_extract(char ***t, const char *s, const char *separators, Extract
                 l[n] = NULL;
         }
 
-        if (!l)
+        if (!l) {
                 l = new0(char*, 1);
+                if (!l)
+                        return -ENOMEM;
+        }
 
         *t = l;
         l = NULL;
 
-        return 0;
+        return (int) n;
 }
 
 char *strv_join(char **l, const char *separator) {
@@ -745,3 +748,41 @@ char **strv_skip(char **l, size_t n) {
 
         return l;
 }
+
+int strv_extend_n(char ***l, const char *value, size_t n) {
+        size_t i, j, k;
+        char **nl;
+
+        assert(l);
+
+        if (!value)
+                return 0;
+        if (n == 0)
+                return 0;
+
+        /* Adds the value value n times to l */
+
+        k = strv_length(*l);
+
+        nl = realloc(*l, sizeof(char*) * (k + n + 1));
+        if (!nl)
+                return -ENOMEM;
+
+        *l = nl;
+
+        for (i = k; i < k + n; i++) {
+                nl[i] = strdup(value);
+                if (!nl[i])
+                        goto rollback;
+        }
+
+        nl[i] = NULL;
+        return 0;
+
+rollback:
+        for (j = k; j < i; i++)
+                free(nl[j]);
+
+        nl[k] = NULL;
+        return NULL;
+}