Custom ports are a kind of port that exposes the C port type interface
directly to Scheme. In this way the full capability of C is available
to Scheme, and also the read and write functions can be tail-called from
Scheme (via port-read / port-write).
* libguile/custom-ports.c:
* libguile/custom-ports.h:
* module/ice-9/custom-ports.scm: New files.
* libguile/init.c:
* libguile/Makefile.am:
* am/bootstrap.am: Add to the build.
* doc/ref/api-io.texi: Update the manual.
