Two pg_rewind patches (auto generate recovery conf and ensure clean shutdown)

Hello, Postgres hackers,

Please see the attached patches.

The first patch adds an option to automatically generate recovery conf
contents in related files, following pg_basebackup. In the patch,
GenerateRecoveryConf(), WriteRecoveryConf() and escape_quotes() are almost
same as them on pg_basebackup. The main difference is due to replication
slot support and code (variables) limit. It seems that we could slightly
refactor later to put some common code into another file after aligning
pg_rewind with pg_basebackup. This was tested manually and was done by
Jimmy (cc-ed), Ashiwin (cc-ed) and me.

Another patch does automatic clean shutdown by running a single mode
postgres instance if the target was not clean shut down since that is
required by pg_rewind. This was manually tested and was done by Jimmy
(cc-ed) and me. I'm not sure if we want a test case for that though.

Thanks.