bind.2, clone.2, dup.2, futex.2, mprotect.2, open_by_handle_at.2, pivot_root.2, selec...

bind.2, clone.2, dup.2, futex.2, mprotect.2, open_by_handle_at.2, pivot_root.2, select_tut.2, shmop.2, timer_create.2, userfaultfd.2, getaddrinfo.3, getaddrinfo_a.3, inet_net_pton.3, malloc_info.3, mbstowcs.3, newlocale.3, posix_spawn.3, pthread_sigmask.3, shm_open.3, strtol.3, inotify.7, user_namespaces.7: Add periods at end of sentences in comments

Try to bring a little more consistency on the use of periods
in comments in example code.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>