Implement freading and fwriting. * lib/freading.c: New file. * lib/freading.h: Likewise. * m4/freading.m4: Likewise. * modules/freading: Likewise. * modules/freading-tests: Likewise. * tests/test-freading.c: Likewise. * lib/fwriting.c: New file. * lib/fwriting.h: Likewise. * m4/fwriting.m4: Likewise. * modules/fwriting: Likewise. * modules/fwriting-tests: Likewise. * tests/test-fwriting.c: Likewise. * MODULES.html.sh (File stream based Input/Output): Mention them.