mail/exim: import exim-4.94+fixes branch as state of 2020.09.09

Used git diffs:

[27/37] Fix spelling of local_part_data in docs and debug output
[27/37] Fix spelling of local_part_data in docs and debug output
[28/37] Fix ${readsocket } eol-replacement. Bug 2630
[29/37] Taint: fix off-by-one in is_tainted(). Bug 2634
[30/37] Build: ifdef guard for EXPERIMENTAL_QUEUEFILE
[31/37] Taint: fix off-by-one in is_tainted(). Bug 2634
[32/37] DANE: force SNI to use $domain. Bug 2265
[33/37] DANE: Fix 2-rcpt message, diff domins case. Bug 2265
[34/37] Fix non-DANE build
[35/37] DANE: Fix 2 messages from queue case
[36/37] Fix non-DANE build

While here, make SPF option turned on by default

MFH: 2020Q3