configure: enable maintainer mode by default

Since libsing is still under heavy development, this seems like a sane
default
Fix crash wrapping Singular lists w/o ring dependent elements

This could e.g. be triggered by "SI_names" which returns a list of
strings, which of course are not ring dependent.
Change Singular interpreter var names to contain a space

Suggested by Hans; such names cannot be entered in Singular library
code, and hence should not cause any clashes.
Clarify configure message when GAP was built without its own GMP
Remove Paul and Steve from credits

At least I see no code contributed by them; it looks like a case of
copy & paste that they were listed in the first place.
Add all source and header files to src/Makefile.am
Fix calling vararg Singular interpreter funcs

Also add test cases that actually exercise this functionality.
It was completely broken before this commit.
Further refactor idhdl code
Use WrapMultiArgs in FuncSI_CallProc
Moved interpreter / library call code to separate C++ source file
Change SingularIdHdl to subclass SingObj