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.

bootstrap.conf

@@ -20,7 +20,6 @@
 avoided_gnulib_modules='
   --avoid=canonicalize-lgpl
   --avoid=dummy
-  --avoid=lock
 '
 
 # These modules are obsolete and can probably be removed soon,
@@ -90,8 +89,10 @@ gnulib_modules="
   fnmatch-gnu
   fopen-safer
   fprintftime
+  freopen
   fseeko
   fsusage
+  fsync
   ftello
   ftruncate
   fts
@@ -201,6 +202,7 @@ gnulib_modules="
   strdup-posix
   strftime
   strpbrk
+  strsignal
   strtoimax
   strtoumax
   strverscmp