doc: add per-module overview.

* build-aux/config.ld.in (boilerplate): Set to true so we can put
a copyright comment at the very top of each file.
* ext/posix/ctype.c, ext/posix/curses.c,
ext/posix/curses/chstr.c, ext/posix/curses/window.c,
ext/posix/dirent.c, ext/posix/errno.c, ext/posix/fcntl.c,
ext/posix/fnmatch.c, ext/posix/getopt.c, ext/posix/glob.c,
ext/posix/grp.c, ext/posix/libgen.c, ext/posix/poll.c,
ext/posix/pwd.c, ext/posix/sched.c, ext/posix/signal.c,
ext/posix/stdio.c, ext/posix/stdlib.c, ext/posix/sys/msg.c,
ext/posix/sys/resource.c, ext/posix/sys/socket.c,
ext/posix/sys/stat.c, ext/posix/sys/statvfs.c,
ext/posix/sys/time.c, ext/posix/sys/times.c,
ext/posix/sys/utsname.c, ext/posix/sys/wait.c,
ext/posix/syslog.c, ext/posix/termio.c, ext/posix/time.c,
ext/posix/unistd.c, ext/posix/utime.c: Move copyright comments
to the very top of the file.  Add a short overview description.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>