use xfreopen in place of unchecked freopen

use xfreopen in place of unchecked freopen

* bootstrap.conf (modules): Add xfreopen.
* src/cat.c (main): Include "xfreopen.h".  Use xfreopen.
* src/cksum.c (cksum): Likewise.
* src/head.c (head_file, main): Likewise.
* src/md5sum.c (digest_file): Likewise.
* src/od.c (open_next_file): Likewise.
* src/split.c (type_undef): Likewise.
* src/sum.c (bsd_sum_file, sysv_sum_file): Likewise.
* src/tac.c (tac_file, main): Likewise.
* src/tail.c (tail_file, main): Likewise.
* src/tee.c (tee_files): Likewise.
* src/tr.c (main): Likewise.
* src/wc.c (wc_file): Likewise.
* po/POTFILES.in: Add lib/xfreopen.c