machined: "machinectl clean" can take a while, do it asynchronously from a background process

This is a follow-up to 5d2036b5f3, and also makes
the "machinectl clean" verb asynchronous, after all it's little more than a
series of image removals.

The changes required to make this happen are a bit more comprehensive as we
need to pass information about deleted images back to the client, as well as
information about the image we failed on if we failed on one. Hence, create a
temporary file in /tmp, serialize that data into, and read it from the parent
after the operation is complete.

src/basic/fileio.c

@@ -1354,3 +1354,44 @@ int link_tmpfile(int fd, const char *path, const char *target) {
 
         return 0;
 }
+
+int read_nul_string(FILE *f, char **ret) {
+        _cleanup_free_ char *x = NULL;
+        size_t allocated = 0, n = 0;
+
+        assert(f);
+        assert(ret);
+
+        /* Reads a NUL-terminated string from the specified file. */
+
+        for (;;) {
+                int c;
+
+                if (!GREEDY_REALLOC(x, allocated, n+2))
+                        return -ENOMEM;
+
+                c = fgetc(f);
+                if (c == 0) /* Terminate at NUL byte */
+                        break;
+                if (c == EOF) {
+                        if (ferror(f))
+                                return -errno;
+                        break; /* Terminate at EOF */
+                }
+
+                x[n++] = (char) c;
+        }
+
+        if (x)
+                x[n] = 0;
+        else {
+                x = new0(char, 1);
+                if (!x)
+                        return -ENOMEM;
+        }
+
+        *ret = x;
+        x = NULL;
+
+        return 0;
+}

src/basic/fileio.h

@@ -86,3 +86,5 @@ int open_tmpfile_unlinkable(const char *directory, int flags);
 int open_tmpfile_linkable(const char *target, int flags, char **ret_path);
 
 int link_tmpfile(int fd, const char *path, const char *target);
+
+int read_nul_string(FILE *f, char **ret);