Make initdb's suggested "pg_ctl start" command line more reliable.

The original coding here was not nearly careful enough about quoting
special characters, and it didn't get corner cases right for constructing
the pg_ctl path either.  Use join_path_components() and appendShellString()
to do it honestly, so that the string will more likely work if blindly
copied-and-pasted.

While at it, teach appendShellString() not to quote strings that clearly
don't need it, so that the output from initdb doesn't become uglier than
it was before in typical cases where quoting is not needed.

Ryan Murphy, reviewed by Michael Paquier and myself

Discussion: <CAHeEsBeAe1FeBypT3E8R1ZVZU0e8xv3A-7BHg6bEOi=jZny2Uw@mail.gmail.com>