Enable profiling options when 'pprof' tag is set (#2813)

This commit enables cpu, mem, block, and mutex profling of the FZF
executable. To support flushing the profiles at program exit it adds
util.AtExit to register "at exit" functions and mandates that util.Exit
is used instead of os.Exit to stop the program.

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

src/tui/light_unix.go

@@ -58,7 +58,7 @@ func openTtyIn() *os.File {
 			}
 		}
 		fmt.Fprintln(os.Stderr, "Failed to open "+consoleDevice)
-		os.Exit(2)
+		util.Exit(2)
 	}
 	return in
 }