Add --sign-module to enable users to mok sign their own modules

Add --sign-module to enable users to mok sign their own modules

Add sign-module option.  Move MOK_CONFIG_TEXT, mok_dir_valid_p, mok_sign_file,
generate_mok from stap-serverd.cxx to cscommon.cxx.  Add sign_module function
to cscommon.cxx. Move MOK_PRIVATE_CERT_NAME, MOK_PRIVATE_CERT_FILE,
MOK_CONFIG_FILE to cscommon.h.  Add report_error parameter to generate_mok,
sign_module, mok_dir_valid_p so they can be called from server or client.  If
sign-module is requested then call sign_module from passes_0_4.  stap-server
continues to mok sign using the same code path.