Continue work on changes to recovery.conf API

Hello all
I would like to continue work on new recovery api proposed in thread [1]. We have some form of consensus but thread has been inactive for a long time and i hope i can help.

I start from last published patch [2] and make some changes:
- updated to current HEAD
- made the patch pass make check-world run
- removed recovery.auto.conf lookup and changed pg_basebackup to append recovery parameters to postgresql.auto.conf in both plain and tar formats
- revert back trigger_file logic, but rename to promote_signal_file. I think here is no reason to make GUC only for directory path with hardcoded file name as was discussed in original thread.

Any feedback is strongly appreciated. Thank you

A brief summary of proposed changes:
- if recovery.conf present during start we report FATAL error about old style config
- recovery mode start if exists file "recovery.signal" in datadir, all old recovery_target_* options are replaced by recovery_target_type and recovery_target_value GUC
- start standby mode - by file "standby.signal"
- if present both - standby wins
- pg_basebackup -R will append primary_conninfo and primary_slot_name to postgresql.auto.conf config
- all new GUC are still PGC_POSTMASTER

PS: i will add an entry to 2018-09 CommitFest when it is become available.

regards, Sergei

[1] https://www.postgresql.org/message-id/flat/CANP8%2BjLO5fmfudbB1b1iw3pTdOK1HBM%3DxMTaRfOa5zpDVcqzew%40mail.gmail.com
[2] https://www.postgresql.org/message-id/CANP8+jJo-LO4xtS7G=iN7PG5o60WeWdKEAn+X+Gnf+RNay5jGQ@mail.gmail.com