Turn the conf/rc into a template.
Remove config.h references.

No longer using autoconf.
Assume existence of __attribute__((noreturn))

It's been in gcc for 21 years..
Rename macro ETCDIR -> ETC_DIR.
Remove unused macro ATJOB_MX
Assume existence of POSIX sysconf() name _SC_LOGIN_NAME_MAX.
Rename PAM macro to be more descriptive.
Ensure POSIX compatible fallback for LOG_CRON.
at: only retain variables whose name consists of alphanumerics and underscores

Since a recent security update[1] bash might export variables named
BASH_FUNC_*() to the environment which the serialization code in at
cannot handle properly.

  [1] <https://www.debian.org/security/2014/dsa-3035>
Prepare to release 3.1.16.
Update autoconf.
Correctly remove optional files, cleanup systemd/service directory.
Merge remote-tracking branch 'origin/master'