Guards for older releases of GnuTLS.

gnutls_sec_param_to_pk_bits() and gnutls_rnd() are both new as of
GnuTLS 2.12.x.  Guard their usage on 2.12.0+ at compile time.

In older versions, the vaguely_random_number() function just immediately
calls the fallback, so it's the same as before this change (just one
extra indirection in the code-path).

Define a constant of 1024 for dh-bits for use in those old releases
where GnuTLS won't tell us how many we should use.

Change the on-disk filename for generated D-H params again, replacing
the -normal with -<bitcount>, so that it's 1024 or whatever, and as
the value changes, Exim will automatically start using the new value.