Implement nested block comments in the backend and in psql.

Implement nested block comments in the backend and in psql.
 Include updates for the comment.sql regression test.
Implement SET SESSION CHARACTERISTICS and SET DefaultXactIsoLevel.
Implement SET SESSION CHARACTERISTICS TRANSACTION COMMIT
 and SET AutoCommit in the parser only.
 Need to add code to actually do something.
Implement WITHOUT TIME ZONE type qualifier.
Define SCHEMA keyword, along with stubbed-out grammar.
Implement "[IN|INOUT|OUT] [varname] type" function arguments
 in parser only; INOUT and OUT throws an elog(ERROR).
Add PATH as a type-specific token, since PATH is in SQL99
 to support schema resource search and resolution.