[kernel] Rework Specific_dir API.

src/kernel_services/cmdline_parameters/parameter_sig.mli

src/kernel_services/plugin_entry_points/kernel.ml

src/kernel_services/plugin_entry_points/plugin.ml

src/plugins/e-acsl/src/libraries/misc.ml

src/plugins/e-acsl/src/main.ml

src/plugins/gui/gtk_helper.ml

src/plugins/wp/ProverErgo.ml

src/plugins/wp/ProverWhy3.ml

src/plugins/wp/driver.mll

src/plugins/wp/register.ml

src/plugins/wp/wp_parameters.ml