Fix some issues and improve psql completion for access methods

Fix some issues and improve psql completion for access methods

The following issues have been spotted:
- CREATE INDEX .. USING suggests both index and table AMs, but it should
consider only index AMs.
- CREATE TABLE .. USING has no completion support.  USING was not being
included in the completion list where it should, and follow-up
suggestions for table AMs have been missing as well.
- CREATE ACCESS METHOD .. TYPE suggests only INDEX, with TABLE missing.

Author: Michael Paquier
Discussion: https://postgr.es/m/20190601191007.GC1905@paquier.xyz