Require :sink and :flusher given together in http-*

If both are missing and :receiver is not given, the default way
(string port) is used.  We don't think there's a use case
that is specifying just either one of :sink and :flusher.
Added generator-find
Reference to generator-find added in gauche.generator doc
Added gbuffer-filter
Small fix in example and test
Experimentally added support for static library
Remove staticinit.c by make clean
Fix the bug in the last fix of rfc.http

Handling of the case when the caller didn't give :receiver argument
was wrong - without the fallback value, the caller needed to give #f
to :receiver explicitly in order to use the adaptor receiver.
Typo fixes
Enhanced support of pseudo record types
Make history vars available in r7rs repl mode
Typo fix in a error message
Some code cleanup
Merge branch 'master' into er-macro
Some code reorganization