Add :pass and :flags to gnutls-boot for :keylist

Add :pass and :flags to gnutls-boot for :keylist

* lisp/net/gnutls.el (gnutls-boot-parameters): Add the keys :pass and
:flags, and update the documentation.
* src/gnutls.c (gnutls-boot): Add the keys :pass and :flags, and
update the documentation.
(syms_of_gnutls): Add the symbols :pass, :flags, and the symbols that
correspond to the enumeration constants of the GnuTLS enum
`gnutls_pkcs_encrypt_flags_t'.
(key_file2_aux): Private helper function that translates a list of
symbols to its corresponding `unsigned int' value of the GnuTLS C enum
`gnutls_pkcs_encrypt_flags_t'.

(Bug#50507)