build: use gnulib's freopen module

build: use gnulib's freopen module

* bootstrap.conf (gnulib_modules): Add freopen, strsignal, fsync.
Exposed via make CFLAGS=-DGNULIB_POSIXCHECK 2>&1 \
|perl -lne '/.* use gnulib module (\S+).*/ and print $1' \
|sort |uniq -c|sort -nr
(avoided_gnulib_modules): Don't avoid the "lock" module.
Now it's required, as a dependency of the strsignal module.