Add --exclude-firmlinks and follow firmlinks by default

What a mess.

https://code.blicky.net/yorhel/ncdu/issues/153#issuecomment-764

doc/ncdu.pod

@@ -179,12 +179,9 @@ this option will not follow symlinks to directories and will count each
 symlinked file as a unique file (i.e. unlike how hard links are handled). This
 is subject to change in later versions.
 
-=item --follow-firmlinks
+=item --exclude-firmlinks
 
-(MacOS only) follow firmlinks. WARNING: This may cause directories to be
-scanned and counted multiple times. Firmlink cycles are current (1.15) not
-detected, so it may also cause ncdu to get stuck in an infinite loop and
-eventually run out of memory.
+(MacOS only) Exclude firmlinks.
 
 =item --exclude-kernfs
 
@@ -429,6 +426,11 @@ Item counts are stored in a signed 32-bit integer without overflow detection.
 If you have a directory with more than 2 billion files, quite literally
 anything can happen.
 
+On macOS 10.15 and later, running ncdu on the root directory without
+`--exclude-firmlinks` may cause directories to be scanned and counted multiple
+times. Firmlink cycles are currently (1.15.1) not detected, so it may also
+cause ncdu to get stuck in an infinite loop and eventually run out of memory.
+
 Please report any other bugs you may find at the bug tracker, which can be
 found on the web site at https://dev.yorhel.nl/ncdu
 

src/main.c

@@ -42,7 +42,7 @@ long update_delay = 100;
 int cachedir_tags = 0;
 int extended_info = 0;
 int follow_symlinks = 0;
-int follow_firmlinks = 0;
+int follow_firmlinks = 1;
 int confirm_quit = 0;
 
 static int min_rows = 17, min_cols = 60;
@@ -139,7 +139,8 @@ static void argv_parse(int argc, char **argv) {
     { 'L', 0, "-L,--follow-symlinks" },
     { 'C', 0, "--exclude-caches" },
     {  2,  0, "--exclude-kernfs" },
-    {  3,  0, "--follow-firmlinks" },
+    {  3,  0, "--follow-firmlinks" }, /* undocumented, this behavior is the current default */
+    {  4,  0, "--exclude-firmlinks" },
     { 's', 0, "--si" },
     { 'Q', 0, "--confirm-quit" },
     { 'c', 1, "--color" },
@@ -173,7 +174,7 @@ static void argv_parse(int argc, char **argv) {
       printf("  --exclude-kernfs           Exclude Linux pseudo filesystems (procfs,sysfs,cgroup,...)\n");
 #endif
 #if HAVE_SYS_ATTR_H && HAVE_GETATTRLIST && HAVE_DECL_ATTR_CMNEXT_NOFIRMLINKPATH
-      printf("  --follow-firmlinks         Follow firmlinks on macOS\n");
+      printf("  --exclude-firmlinks        Exclude firmlinks on macOS\n");
 #endif
       printf("  --confirm-quit             Confirm quitting ncdu\n");
       printf("  --color SCHEME             Set color scheme (off/dark)\n");
@@ -204,19 +205,26 @@ static void argv_parse(int argc, char **argv) {
       cachedir_tags = 1;
       break;
 
-    case  2 :
+    case  2 : /* --exclude-kernfs */
 #if HAVE_LINUX_MAGIC_H && HAVE_SYS_STATFS_H && HAVE_STATFS
       exclude_kernfs = 1; break;
 #else
       fprintf(stderr, "This feature is not supported on your platform\n");
       exit(1);
 #endif
-    case  3 :
+    case  3 : /* --follow-firmlinks */
 #if HAVE_SYS_ATTR_H && HAVE_GETATTRLIST && HAVE_DECL_ATTR_CMNEXT_NOFIRMLINKPATH
       follow_firmlinks = 1; break;
 #else
       fprintf(stderr, "This feature is not supported on your platform\n");
       exit(1);
+#endif
+    case  4 : /* --exclude-firmlinks */
+#if HAVE_SYS_ATTR_H && HAVE_GETATTRLIST && HAVE_DECL_ATTR_CMNEXT_NOFIRMLINKPATH
+      follow_firmlinks = 0; break;
+#else
+      fprintf(stderr, "This feature is not supported on your platform\n");
+      exit(1);
 #endif
     case 'c':
       if(strcmp(val, "off") == 0)  { uic_theme = 0; }