Patch: initdb: "'" for QUOTE_PATH (non-windows)

Hello Postgres Team!

This is to fix an issue that came up for me when running initdb.

At the end of a successful initdb it says:

Success. You can now start the database server using:
pg_ctl -D /some/path/to/data -l logfile start

but this command did not work for me because my data directory
contained a space. The src/bin/initdb/initdb.c source code
did already have a QUOTE_PATH constant, but it was the empty
string for non-windows cases.

Therefore, added quotes via existing QUOTE_PATH constant:

pg_ctl -D '/some/path/to/data' -l logfile start