Tightening isspace() tests where behavior should match SQL parser

The proximate cause of bug #14662,
https://www.postgresql.org/message-id/flat/20170519162316.29945.5021%40wrigleys.postgresql.org
appears to be that SplitIdentifierString's isspace() checks are
identifying some bytes of a multibyte character as spaces.  It's not
quite clear to me whether there's an encoding configuration problem
involved in this specific report, but in any case feeding individual
bytes of a multibyte character to <ctype.h> functions is highly dubious.
The best you can say about that is that the behavior will be
platform-dependent.

I think that the easiest way to resolve this is to replace the isspace()
calls with scanner_isspace(), on the grounds that we are trying to parse
identifiers the same way that the core SQL lexer would, and therefore we
should use its definition of whitespace.

I went looking through all our other calls of isspace() to see if we
have the same problem anywhere else, and identified these functions
as being at risk:

parse_ident()
regproc.c's parseNameAndArgTypes (for regprocedurein and siblings)
SplitIdentifierString()
SplitDirectoriesString()

In all four cases, we must allow for non-ASCII input and it's
arguable that correct behavior is to match the core lexer,
so I propose replacing isspace() with scanner_isspace() in
these places.

There are quite a lot more isspace() calls than that of course,
but the rest of them seem probably all right to me.  As an
example, I don't see a need to make float8in() stricter about
what it will allow as trailing whitespace.  We're not expecting
any non-ASCII input really, and if we do see multibyte characters
and all the bytes manage to pass isspace(), not much harm is done.

Attached is a proposed patch.  I'm vacillating on whether to
back-patch this --- it will fix a reported bug, but it seems
at least somewhat conceivable that it will also break cases
that were working acceptably before.  Thoughts?

			regards, tom lane