Abort more noisily

cmirror may abort on various error conditions. There was a complaint,
that the error mesasge might be overlooked because of the normal
diagnostic output:

    buczek@theinternet:~$ cmirror /tmp/a /tmp/b --delete
    theinternet.molgen.mpg.de: creating empty ./1
    theinternet.molgen.mpg.de: creating empty ./10
    theinternet.molgen.mpg.de: creating empty ./2
    theinternet.molgen.mpg.de: creating empty ./3
    theinternet.molgen.mpg.de: creating empty ./4
    theinternet.molgen.mpg.de: creating empty ./5
    theinternet.molgen.mpg.de: creating empty ./6
    theinternet.molgen.mpg.de: creating empty ./7
    theinternet.molgen.mpg.de: creating empty ./8
    theinternet.molgen.mpg.de: creating empty ./9
    theinternet.molgen.mpg.de: rm -r ./xx
    rm: cannot remove './xx/aa': Permission denied
    buczek@theinternet:~$

To make this more clear, an additional diagnostic output was requested
to show that cmirror didn't finish its job:

    buczek@theinternet:~$ cmirror /tmp/a /tmp/b --delete
    theinternet.molgen.mpg.de: creating empty ./1
    theinternet.molgen.mpg.de: creating empty ./10
    theinternet.molgen.mpg.de: creating empty ./2
    theinternet.molgen.mpg.de: creating empty ./3
    theinternet.molgen.mpg.de: creating empty ./4
    theinternet.molgen.mpg.de: creating empty ./5
    theinternet.molgen.mpg.de: creating empty ./6
    theinternet.molgen.mpg.de: creating empty ./7
    theinternet.molgen.mpg.de: creating empty ./8
    theinternet.molgen.mpg.de: creating empty ./9
    theinternet.molgen.mpg.de: rm -r ./xx
    rm: cannot remove './xx/aa': Permission denied
    cmirror aborted
    buczek@theinternet:~$

Implementation:

- Add code to `die()` to optionally show the additional
  message. This can be selected by a global flag

- Let the master set that flag unless `--quiet` is selected

- Add code to `die()` so that it can be called with `die(NULL)` to show
  only the optional additional message but no specific error message.

- Replace `_exit(1)` with `die(NULL)` n places where it was used to
  abort the parent process after a child process, which as the duty to
  emit the error messages for its operation, has failed.

cmirror.c

@@ -34,6 +34,7 @@
 #define STEAL_POINTER(SRC) ({ void *tmp=*(SRC); *(SRC)=NULL; tmp; })
 
 static const char *log_prefix;
+static int noisy_abort;
 
 static void warn(const char *restrict fmt, ...) {
     if (log_prefix)
@@ -45,12 +46,16 @@ static void warn(const char *restrict fmt, ...) {
 }
 
 static G_NORETURN void die(const char *restrict fmt, ...) {
-    if (log_prefix)
-        fprintf(stderr, "%s: ", log_prefix);
-    va_list ap;
-    va_start(ap, fmt);
-    vfprintf(stderr, fmt, ap);
-    va_end(ap);
+    if (fmt) {
+        if (log_prefix)
+            fprintf(stderr, "%s: ", log_prefix);
+        va_list ap;
+        va_start(ap, fmt);
+        vfprintf(stderr, fmt, ap);
+        va_end(ap);
+    }
+    if (noisy_abort)
+        fputs("cmirror aborted\n", stderr);
     _exit(1);
 }
 
@@ -355,7 +360,7 @@ static void execvp_checked(char **argv) {
     int wstatus;
     waitpid(pid, &wstatus, 0);
     if (wstatus)
-        _exit(1);
+        die(NULL);
 }
 
 static void fileop_rmdir_recurse(char *path) {
@@ -674,7 +679,7 @@ static void slave(char *slave_path) {
         if (error)
             die("mkdir: %s\n", error->message);
         if (wait_status)
-            _exit(1);
+            die(NULL);
     }
     int res = chdir(slave_path);
     if (res == -1)
@@ -1084,6 +1089,9 @@ static void master(char *master_path, char *target) {
     g_autofree char *slave_path;
     g_autofree char *slave_user;
 
+    if (!quiet)
+        noisy_abort = 1;
+
     EXCEPTS = g_ptr_array_new();
     g_ptr_array_add (EXCEPTS, "./quota.group");
     g_ptr_array_add (EXCEPTS, "./quota.user");
@@ -1435,7 +1443,7 @@ static void master(char *master_path, char *target) {
     if (!res)
         die("%m");
     if (wstatus) {
-        _exit(1);
+        die(NULL);
     }
 }