Ship default configuration /etc/exim/exim.conf as well as
a simple procd init script. Enable building with LMTP for better
integration with dovecot.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

Several exploitable vulnerabilities in Exim were reported to us and are
fixed.
Local vulnerabilities
- CVE-2020-28007: Link attack in Exim's log directory
- CVE-2020-28008: Assorted attacks in Exim's spool directory
- CVE-2020-28014: Arbitrary PID file creation
- CVE-2020-28011: Heap buffer overflow in queue_run()
- CVE-2020-28010: Heap out-of-bounds write in main()
- CVE-2020-28013: Heap buffer overflow in parse_fix_phrase()
- CVE-2020-28016: Heap out-of-bounds write in parse_fix_phrase()
- CVE-2020-28015: New-line injection into spool header file (local)
- CVE-2020-28012: Missing close-on-exec flag for privileged pipe
- CVE-2020-28009: Integer overflow in get_stdinput()
Remote vulnerabilities
- CVE-2020-28017: Integer overflow in receive_add_recipient()
- CVE-2020-28020: Integer overflow in receive_msg()
- CVE-2020-28023: Out-of-bounds read in smtp_setup_msg()
- CVE-2020-28021: New-line injection into spool header file (remote)
- CVE-2020-28022: Heap out-of-bounds read and write in extract_option()
- CVE-2020-28026: Line truncation and injection in spool_read_header()
- CVE-2020-28019: Failure to reset function pointer after BDAT error
- CVE-2020-28024: Heap buffer underflow in smtp_ungetc()
- CVE-2020-28018: Use-after-free in tls-openssl.c
- CVE-2020-28025: Heap out-of-bounds read in pdkim_finish_bodyhash()

The update to 4.94.2 also integrates a fix for a printf format issue
previously addressed by a local patch which is removed.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

Signed-off-by: Rosen Penev <rosenp@gmail.com>

CentOS 7 has an old GCC that does not default to c11.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Add Exim MTA variants:
 * exim
   plain variant without any TLS library which hence comes
   without TLS, DANE and DKIM.
 * exim-openssl
   linked against libopenssl
 * exim-gnutls
   linked against libgnutls
 * exim-ldap
   linked against libopenssl, libopenldap and libsasl2

Provide packages for lookup modules
 * cdb
 * dbmdb
 * dnsdb
 * json (depends on jansson)
 * mysql (depends on libmariadb)
 * passwd
 * pgsql (depends on libpq)
 * redis (depends on libhiredis)
 * sqlite (depends on libsqlite3)

Note:
As gnutls requires libunbound which depends on libopenssl to provide
libgnutls-dane, disable DANE by default when building with gnutls.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>