pkcs11: read pkcs11.conf at once with gnutls_load_file

pkcs11: read pkcs11.conf at once with gnutls_load_file

clang-analyzer from Clang 19 complains about the fgets usage while a
mutex is held:

  pkcs11.c:911:9: warning: Call to blocking function 'fgets' inside of critical section [unix.BlockInCriticalSection]
    911 |         while (fgets(line, sizeof(line), fp) != NULL) {
        |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This changes the logic to read the content of the file at once to
avoid iterative calls to fgets.

Signed-off-by: Daiki Ueno <ueno@gnu.org>