* module/ice-9/boot-9.scm (%cond-expand-features): Remove redundant list
of feature identifiers in the comment. Explain more clearly what
belongs in this list. Remove srfi-6.
* module/srfi/srfi-4.scm, module/srfi/srfi-27.scm,
module/srfi/srfi-31.scm, module/srfi/srfi-38.scm,
module/srfi/srfi-39.scm, module/srfi/srfi-42.scm,
module/srfi/srfi-45.scm, module/srfi/srfi-67.scm: Add missing
'cond-expand-provide'.
* module/srfi/srfi-69.scm: Fix erroneous 'cond-expand-provide'.
* doc/ref/srfi-modules.texi (SRFI-0): Update the list of features in
Guile core.
* libguile/ports.c (scm_init_ports): Export the port fluids to Scheme,
temporarily.
* module/ice-9/boot-9.scm (fluid->parameter): Turn `current-input-port'
et al into srfi-39 parameters, backed by the exported fluids, then
remove the fluids from the guile module.
(%cond-expand-features): Add srfi-39.
* module/srfi/srfi-39.scm: Re-export features from boot-9.
* test-suite/tests/parameters.test: Add tests.
