Check for NCDU_SHELL environment variable when spawning shell

Check if the environment variable NCDU_SHELL is defined before the SHELL
variable is checked. This makes it possible to specify a program to
execute when 'b' is pressed. Setting SHELL to for example "mc" (Midnight
Commander) didn't work because mc already uses SHELL to execute
commands.

doc/ncdu.pod

@@ -214,9 +214,15 @@ Refresh/recalculate the current directory.
 
 Spawn shell in current directory.
 
-We first check the $SHELL environment variable of the user for the preferred
-shell interpreter. If it's not set, we fall back to the compile time
-configured default shell (usually /bin/bash).
+We first check the $NCDU_SHELL environment variable of the user for the
+preferred shell interpreter. If it's not set, $SHELL is checked. If that
+variable also isn't defined, we fall back to the compile time configured
+default shell (usually /bin/bash). This allows you to specify which program to
+launch when 'b' is pressed. For example, if you want ncdu to execute the
+Midnight Commander file manager in the current directory:
+
+  export NCDU_SHELL=mc
+  ncdu
 
 =item q
 

src/shell.c

@@ -52,9 +52,12 @@ void shell_draw() {
            "Press any key to continue.",
            full_path, res);
   } else {
-    char *shell = getenv("SHELL");
+    char *shell = getenv("NCDU_SHELL");
+    if (shell == NULL) {
+      shell = getenv("SHELL");
       if (shell == NULL)
         shell = DEFAULT_SHELL;
+    }
 
     res = system(shell);