[kernel] simplifies parameters for directories

src/kernel_services/ast_queries/cil_builtins.ml

src/kernel_services/ast_queries/file.ml

src/kernel_services/cmdline_parameters/parameter_sig.ml

src/kernel_services/plugin_entry_points/plugin.ml

src/plugins/e-acsl/src/project_initializer/rtl.ml

src/plugins/gui/gtk_helper.ml

src/plugins/wp/ProverWhy3.ml

src/plugins/wp/driver.mll

tests/libc/check_parsing_individual_headers.ml

tests/misc/share_directory.t/directories.ml

tests/misc/share_directory.t/run.t

tests/misc/user_directories.unix.t/directories.ml

tests/misc/user_directories.unix.t/run.t