Bug#21025 (misleading error message when creating functions named 'x', or 'y')
Bug#22619 (Spaces considered harmful)

This change contains a fix to report warnings or errors, and multiple tests
cases.

Before this fix, name collisions between:
- Native functions
- User Defined Functions
- Stored Functions
were not systematically reported, leading to confusing behavior.

I) Native / User Defined Function

Before this fix, is was possible to create a UDF named "foo", with the same
name as a native function "foo", but it was impossible to invoke the UDF,
since the syntax "foo()" always refer to the native function.
After this fix, creating a UDF fails with an error if there is a name
collision with a native function.

II) Native / Stored Function

Before this fix, is was possible to create a SF named "db.foo", with the same
name as a native function "foo", but this was confusing since the syntax
"foo()" would refer to the native function. To refer to the Stored Function,
the user had to use the "db.foo()" syntax.
After this fix, creating a Stored Function reports a warning if there is a
name collision with a native function.

III) User Defined Function / Stored Function

Before this fix, creating a User Defined Function "foo" and a Stored Function
"db.foo" are mutually exclusive operations. Whenever the second function is
created, an error is reported. However, the test suite did not cover this
behavior.
After this fix, the  behavior is unchanged, and is now covered by test cases.

Note that the code change in this patch depends on the fix for Bug 21114.