Added tls_dh_max_bits & check tls_require_ciphers early.

Janne Snabb tracked down the GnuTLS 2.12 vs NSS (Thunderbird) interop
problems to a hard-coded limit of 2236 bits for DH in NSS while GnuTLS
was suggesting 2432 bits as normal.

Added new global option tls_dh_max_bits to clamp all DH values (client
or server); unexpanded integer.  Default value to 2236.  Apply to both
GnuTLS and OpenSSL (which requires tls_dh_params for this).

Tired of debugging "SMTP fails TLS" error messages in mailing-lists
caused by OpenSSL library/include clashes, and of finding out I typo'd
in tls_require_ciphers only at the STARTTLS handshake.  During readconf,
fork/drop-privs/initialise-TLS-library.  In that, if tls_require_ciphers
is set, then validate it.

The validation child will panic if it can't initialise or if
tls_require_ciphers can't be parsed, else it exits 0.  If the child
exits anything other than 0, the main Exim process will exit.