core: Implement timeout based umount/remount limit

Remount, and subsequent umount, attempts can hang for inaccessible network
based mount points. This can leave a system in a hard hang state that
requires a hard reset in order to recover. This change moves the remount,
and umount attempts into separate child processes. The remount and umount
operations will block for up to 90 seconds (DEFAULT_TIMEOUT_USEC). Should
those waits fail, the parent will issue a SIGKILL to the child and continue
with the shutdown efforts.

In addition, instead of only reporting some additional errors on the final
attempt, failures are reported as they occur.

src/basic/process-util.h

@@ -33,6 +33,7 @@
 #include "format-util.h"
 #include "ioprio.h"
 #include "macro.h"
+#include "time-util.h"
 
 #define procfs_file_alloca(pid, field)                                  \
         ({                                                              \
@@ -61,6 +62,7 @@ int get_process_ppid(pid_t pid, pid_t *ppid);
 
 int wait_for_terminate(pid_t pid, siginfo_t *status);
 int wait_for_terminate_and_warn(const char *name, pid_t pid, bool check_exit_code);
+int wait_for_terminate_with_timeout(pid_t pid, usec_t timeout);
 
 void sigkill_wait(pid_t pid);
 void sigkill_waitp(pid_t *pid);