gdesktopappinfo: enable fast posix_spawn gspawn codepath

In order to use the new posix_spawn gspawn codepath, for more robust
app launching when available memory is low, we need to meet some
conditions.

child_setup needs to be NULL for this optimization to work, so drop
the internal child_setup that is used here. Replace it with a lightweight
wrapper binary (gio-launch-desktop) that sets GIO_LAUNCHED_DESKTOP_FILE_PID
before executing the app.

Adjust PATH for gio tests so that it can execute the new binary from the
build directory.

gio/meson.build

@@ -411,6 +411,12 @@ if host_system != 'windows'
     contenttype_sources += files('gcontenttype.c')
     appinfo_sources += files('gdesktopappinfo.c')
     gio_unix_include_headers += files('gdesktopappinfo.h')
+
+    executable('gio-launch-desktop', 'gio-launch-desktop.c',
+      install : true,
+      c_args : gio_c_args,
+      # intl.lib is not compatible with SAFESEH
+      link_args : noseh_link_args)
   endif
 
   subdir('xdgmime')