All GNU packages now use ftp.gnu.org (stable canonical server) as the
primary filelist source, with ftpmirror.gnu.org configured as
source-mirror fallback. ftpmirror uses DNS round-robin across many
mirrors of varying reliability; this setup eliminates random CI failures
caused by unreachable mirror nodes while preserving a fallback path.
ftp.gnu.org is unreliable and frequently times out during CI builds.
ftpmirror.gnu.org is GNU's own CDN that auto-redirects to the nearest
mirror. This is the recommended download method per
https://www.gnu.org/prep/ftp.en.html
Also normalizes /pub/gnu/ paths to /gnu/ since ftpmirror only serves
the latter.
Affects: libiconv, gettext, gmp, idn2, libunistring, ncurses, readline
