WIP psql \df choose functions by their arguments

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Improve psql \df to choose functions by their arguments

== OVERVIEW

Having to scroll through same-named functions with different argument types 
when you know exactly which one you want is annoying at best, error causing 
at worst. This patch enables a quick narrowing of functions with the 
same name but different arguments. For example, to see the full details 
of a function names "myfunc" with a TEXT argument, but not showing 
the version of "myfunc" with a BIGINT argument, one can now do:

psql=# \df myfunc text

For this, we are fairly liberal in what we accept, and try to be as 
intuitive as possible.

Features:

* Type names are case insensitive. Whitespace is optional, but quoting is respected:

greg=# \df myfunc text "character varying" INTEGER

* Abbreviations of common types is permitted (because who really likes 
to type out "character varying"?), so the above could also be written as:

greg=# \df myfunc text varchar int

* The matching is greedy, so you can see everything matching a subset:

greg=# \df myfunc timestamptz
                                   List of functions
 Schema |  Name  | Result data type |            Argument data types            | Type 
- --------+--------+------------------+-------------------------------------------+------
 public | myfunc | void             | timestamp with time zone                  | func
 public | myfunc | void             | timestamp with time zone, bigint          | func
 public | myfunc | void             | timestamp with time zone, bigint, boolean | func
 public | myfunc | void             | timestamp with time zone, integer         | func
 public | myfunc | void             | timestamp with time zone, text, cidr      | func
(5 rows)

* The appearance of a closing paren indicates we do not want the greediness:

greg=# \df myfunc (timestamptz, bigint)
                              List of functions
 Schema |  Name  | Result data type |       Argument data types        | Type 
- --------+--------+------------------+----------------------------------+------
 public | myfunc | void             | timestamp with time zone, bigint | func
(1 row)


== TAB COMPLETION:

I'm not entirely happy with this, but I figure piggybacking 
onto COMPLETE_WITH_FUNCTION_ARG is better than nothing at all.
Ideally we'd walk prev*_wd to refine the returned list, but 
that's an awful lot of complexity for very little gain, and I think 
the current behavior of showing the complete list of args each time 
should suffice.


== DOCUMENTATION:

The new feature is briefly mentioned: wordsmithing help in the 
sgml section is appreciated. I'm not sure how many of the above features 
need to be documented in detail.

Regarding psql/help.c, I don't think this really warrants a change there. 
As it is, we've gone through great lengths to keep this overloaded backslash 
command left justified with the rest!


== TESTS:

I put this into psql.c, seems the best place. Mostly testing out 
basic functionality, quoting, and the various abbreviations. Not much 
else to test, near as I can tell, as this is a pure convienence addition 
and shouldn't affect anything else. Any extra words after a function name 
for \df was previously treated as an error.


== IMPLEMENTATION:

Rather than messing with psqlscanslash, we simply slurp in the entire rest 
of the line via psql_scan_slash_option (all of which was previously ignored). 
This is passed to describeFunction, which then uses strtokx to break it 
into tokens. We look for a match by comparing the current proargtypes entry, 
casted to text, against the lowercase version of the token found by strtokx. 
Along the way, we convert things like "timestamptz" to the official version 
(i.e. "timestamp with time zone"). If any of the tokens start with a closing 
paren, we immediately stop parsing and set pronargs to the current number 
of valid tokens, thereby forcing a match to one (or zero) functions.

dcd972f6b945070ef4454ea39d25378427a90e89  df.patch

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