Fix #81: GUI programs shouldn't exit on closing the parent terminal

Issue reproduction steps:

1. Spawn a GUI program e.g. open a PDF file in zathura or evince.
2. Without quitting `nnn` close the terminal.
3. Notice that the application quits too.

Can be a nagging issue if someone is not using a drop-down terminal.

Fix: detach a GUI child and start it in a new session.

There are 2 aspects to this commit:

- It fixes #81: in cases where we do not wait for a spawned child we can assume
that the child is a GUI process. We detach and spawn the child in a new session.
- It changes the behaviour to @Rahi374's suggestion in PR #40 i.e. invoke the DE
opener in F_NOWAIT mode.

README.md

@@ -74,8 +74,6 @@ Have fun with it! PRs are welcome. Check out [#1](https://github.com/jarun/nnn/i
   - [boost chdir prompt](#boost-chdir-prompt)
   - [set idle timeout](#set-idle-timeout)
   - [show hot plugged drives](#show-hot-plugged-drives)
-- [Troubleshooting](#troubleshooting)
-  - [nnn blocks on opening files](#nnn-blocks-on-opening-files)
 - [Why fork?](#why-fork)
 - [Mentions](#mentions)
 - [Developers](#developers)
@@ -453,14 +451,6 @@ The terminal screensaver is disabled by default. To set the wait time in seconds
 
 Enable volume management in your DE file manager and set removable drives or media to be auto-mounted when inserted. Then visit the usual mount point location (`/mnt` or `/media/user`) in `nnn`.
 
-### Troubleshooting
-
-#### nnn blocks on opening files
-
-Ideally nnn should not block. Unfortunately, sometimes even the same desktop opener behaves differently on different Linux desktop environments. If `nnn` does block when a file is open, set the environment variable `NNN_NOWAIT` to any non-zero value. For example,
-
-    export NNN_NOWAIT=1
-
 ### Why fork?
 
 I chose to fork because:

nnn.1

@@ -253,11 +253,6 @@ screensaver.
     export NNN_NO_X=1
 .Ed
 .Pp
-\fBNNN_NOWAIT:\fR necessary only if nnn blocks while a file is open.
-.Bd -literal
-    export NNN_NOWAIT=1
-.Ed
-.Pp
 \fBNNN_QUOTE_ON:\fR wrap copied paths within single quotes. Useful for pasting
 names in the shell.
 .Sh KNOWN ISSUES

nnn.c

@@ -248,7 +248,6 @@ static char *player;
 static char *copier;
 static char *editor;
 static char *desktop_manager;
-static char nowait = F_NOTRACE;
 static blkcnt_t ent_blocks;
 static blkcnt_t dir_blocks;
 static ulong num_files;
@@ -759,6 +758,12 @@ spawn(const char *file, const char *arg1, const char *arg2, const char *dir, uch
 			close(fd);
 		}
 
+		if (flag & F_NOWAIT) {
+			signal(SIGHUP, SIG_IGN);
+			signal(SIGPIPE, SIG_IGN);
+			setsid();
+		}
+
 		if (flag & F_SIGINT)
 			signal(SIGINT, SIG_DFL);
 		execlp(file, file, arg1, arg2, NULL);
@@ -2520,7 +2525,7 @@ nochange:
 				}
 
 				/* Invoke desktop opener as last resort */
-				spawn(utils[OPENER], newpath, NULL, NULL, nowait);
+				spawn(utils[OPENER], newpath, NULL, NULL, F_NOWAIT | F_NOTRACE);
 				continue;
 			}
 			default:
@@ -2878,7 +2883,7 @@ nochange:
 				goto nochange;
 			}
 
-			spawn(desktop_manager, path, NULL, path, F_NOTRACE | F_NOWAIT);
+			spawn(desktop_manager, path, NULL, path, F_NOWAIT | F_NOTRACE);
 			break;
 		case SEL_FSIZE:
 			cfg.sizeorder ^= 1;
@@ -3341,9 +3346,6 @@ main(int argc, char *argv[])
 	/* Get the default copier, if set */
 	copier = getenv("NNN_COPIER");
 
-	/* Get nowait flag */
-	nowait |= getenv("NNN_NOWAIT") ? F_NOWAIT : 0;
-
 	/* Enable quotes if opted */
 	if (getenv("NNN_QUOTE_ON"))
 		cfg.quote = 1;