Clean up use_posix_spawn

Switch from a global variable to a real function. Make the value atomic.
Clean up handle_fish_use_posix_spawn_change().

src/env_dispatch.cpp

@@ -260,11 +260,18 @@ static void handle_curses_change(const environment_t &vars) {
     init_curses(vars);
 }
 
+/// Whether to use posix_spawn when possible.
+static relaxed_atomic_bool_t g_use_posix_spawn{false};
+
+bool get_use_posix_spawn() { return g_use_posix_spawn; }
+
 static void handle_fish_use_posix_spawn_change(const environment_t &vars) {
-    // note this defaults to true
-    auto use_posix_spawn = vars.get(L"fish_use_posix_spawn");
-    g_use_posix_spawn =
-        use_posix_spawn.missing_or_empty() ? true : bool_from_string(use_posix_spawn->as_string());
+    // Note if the variable is missing or empty, we default to true.
+    if (auto var = vars.get(L"fish_use_posix_spawn")) {
+        g_use_posix_spawn = var->empty() || bool_from_string(var->as_string());
+    } else {
+        g_use_posix_spawn = true;
+    }
 }
 
 /// Allow the user to override the limit on how much data the `read` command will process.
@@ -570,9 +577,6 @@ static void init_locale(const environment_t &vars) {
 /// Returns true if we think the terminal supports setting its title.
 bool term_supports_setting_title() { return can_set_term_title; }
 
-/// Miscellaneous variables.
-bool g_use_posix_spawn = false;
-
 // Limit `read` to 100 MiB (bytes not wide chars) by default. This can be overridden by the
 // fish_read_limit variable.
 size_t read_byte_limit = 100 * 1024 * 1024;