This prctl(2) option can be used to set the "child subreaper" attribute
of the calling process.  This allows a process to take on the role of
'init', which means the process will inherit descendant processes when
their immediate parent terminates.

This will be used in an upcoming commit that uses a double fork(2) +
unshare(2) to create a new PID namespace.  The parent from the second
fork will terminate leaving the child process to be inherited by 'init'.
Aside from it being better to maintain the parent/child relationships
between the various unit processes, without setting this you need to ^C
twice to fully quit unit when running in the foreground after the double
fork.

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Due to the need to replace our use of clone/__NR_clone on Linux with
fork(2)/unshare(2) for enabling Linux namespaces(7) to keep the
pthreads(7) API working.  Let's rename NXT_HAVE_CLONE to
NXT_HAVE_LINUX_NS, i.e name it after the feature, not how it's
implemented, then in future if we change how we do namespaces again we
don't have to rename this.

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

This prctl(2) option is checked for in auto/isolation, unfortunately due
to a typo this feature has never been enabled.

In the auto/isolation script the feature name was down as
NXT_HAVE_PR_SET_NO_NEW_PRIVS0, which means we end up with the following
in build/nxt_auto_config.h

  #ifndef NXT_HAVE_PR_SET_NO_NEW_PRIVS0
  #define NXT_HAVE_PR_SET_NO_NEW_PRIVS0  1
  #endif

Whereas everywhere else is checking for NXT_HAVE_PR_SET_NO_NEW_PRIVS.

This also guards the inclusion of sys/prctl.h in src/nxt_process.c which
is required by a subsequent commit.

Fixes: e2b53e1 ("Added "rootfs" feature.")
Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

In configure we set NXT_HAVE_LITTLE_ENDIAN for i386, amd64 and x86_64.
However that misses at least AArch64 (arm64) where it's usually run in
little endian mode.

However none of that really matters as NXT_HAVE_LITTLE_ENDIAN isn't used
anywhere.  So why this patch?

The only place we need to explicitly know about endianness is the
nxt_websocket_header_t structure where we lay it out differently
depending on endianness.

This is currently done using BYTE_ORDER, LITTLE_ENDIAN and BIG_ENDIAN
macros.

However on at least illumos (OpenSolaris / OpenIndiana) those macros are
not defined and we get compiler errors due to duplicate structure
members.

So let's use our own NXT_HAVE_{BIG,LITTLE}_ENDIAN macros.  However it
would be better to detect endianness programmatically as some
architectures can run in either mode, e.g Linux used to run in big
endian on PowerPC but has since switched to little endian (to match
x86).

This commit adds an auto/endian script (using a slightly modified
version of the test program from nginx's auto script), that checks for
the endianness of the platform being built on.  E.g

  checking for endianness ... little endian

The next commit will switch the nxt_websocket_header_t structure over to
these new macros.

Link: <https://github.com/nginx/unit/pull/298>
Link: <https://developer.ibm.com/articles/l-power-little-endian-faq-trs/>
Tested-by: Alejandro Colomar <alx@nginx.com>
Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

This commit enables the building of the cgroup code. This is only built
when the cgroupv2 filesystem is found.

If cgroupv2 support is found then

  cgroupv2: .................. YES

will be printed by ./configure

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

It's for the introduction of njs support.
For each option that supports native variable and JS template literals introduced next,
it's unified as template string.

No functional changes.