import: add image verification using gpg

This also adds an initial keyring for the verification, that contains
Ubuntu's and Fedora's key. We should probably add more entries sooner or
later.

src/shared/util.c

@@ -8011,3 +8011,13 @@ ssize_t sparse_write(int fd, const void *p, size_t sz, size_t run_length) {
 
         return q - (const uint8_t*) p;
 }
+
+void sigkill_wait(pid_t *pid) {
+        if (!pid)
+                return;
+        if (*pid <= 1)
+                return;
+
+        if (kill(*pid, SIGKILL) > 0)
+                (void) wait_for_terminate(*pid, NULL);
+}

src/shared/util.h

@@ -1065,3 +1065,6 @@ void release_lock_file(LockFile *f);
 #define RLIMIT_MAKE_CONST(lim) ((struct rlimit) { lim, lim })
 
 ssize_t sparse_write(int fd, const void *p, size_t sz, size_t run_length);
+
+void sigkill_wait(pid_t *pid);
+#define _cleanup_sigkill_wait_ _cleanup_(sigkill_wait)