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.

@alejandro-colomar reported that the build was broken on MacOS

cc  -o build/unitd -pipe -fPIC -fvisibility=hidden -O -W -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -fstrict-aliasing -Wstrict-overflow=5 -Wmissing-prototypes -Werror -g   \
		build/src/nxt_main.o  build/libnxt.a \
		    \
                      \
                    -L/usr/local/Cellar/pcre2/10.40/lib -lpcre2-8
Undefined symbols for architecture x86_64:
  "_nxt_fs_mkdir_parent", referenced from:
      _nxt_runtime_pid_file_create in libnxt.a(nxt_runtime.o)
      _nxt_runtime_controller_socket in libnxt.a(nxt_controller.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [build/unitd] Error 1

This was due to commit 57fc920 ("Socket: Created control socket & pid file
directories.").

This happened because this commit introduced the usage of
nxt_fs_mkdir_parent() in core code which uses nxt_fs_mkdir(), both of
these are defined in src/nxt_fs.c. It turns out however that this file
doesn't get built on MacOS (or any system that isn't Linux or that
lacks a FreeBSD compatible nmount(2) system call) due to the following

In auto/sources we have

  if [ $NXT_HAVE_ROOTFS = YES ]; then
      NXT_LIB_SRCS="$NXT_LIB_SRCS src/nxt_fs.c"
  fi

NXT_HAVE_ROOTFS is set in auto/isolation

  If [ $NXT_HAVE_MOUNT = YES -a $NXT_HAVE_UNMOUNT = YES ]; then
      NXT_HAVE_ROOTFS=YES

      cat << END >> $NXT_AUTO_CONFIG_H
  #ifndef NXT_HAVE_ISOLATION_ROOTFS
  #define NXT_HAVE_ISOLATION_ROOTFS  1
  #endif
  END

  fi

While we do have a check for a generic umount(2) which is found on
MacOS, for mount(2) we currently only check for the Linux mount(2) and
FreeBSD nmount(2) system calls. So NXT_HAVE_ROOTFS is set to NO on MacOS
and we don't build src/nxt_fs.c

This fixes the immediate build issue by taking the mount/umount OS
support out of nxt_fs.c into a new nxt_fs_mount.c file which is guarded
by the above while we now build nxt_fs.c unconditionally.

This should fix the build on any _supported_ system.

Reported-by: Alejandro Colomar <alx@nginx.com>
Fixes: 57fc920 ("Socket: Created control socket & pid file directories.")
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

nxt_sockaddr_ntop() stopped being used in commit (git) 029942f4eb71.
It has been replaced mostly by nxt_sockaddr_text().

    commit 029942f4eb7196c2cff0d0e26bc6ff274138f7d8
    Author: Igor Sysoev <igor@sysoev.ru>
    Date:   Wed Feb 22 15:09:59 2017 +0300

        I/O operations refactoring.

nxt_job_sockaddr_parse() stopped being used in commit (git) 794248090a74.

    commit 794248090a74f31cbfcf24ea8c835df2d4d21073
    Author: Igor Sysoev <igor@sysoev.ru>
    Date:   Wed Mar 4 14:04:08 2020 +0300

        Legacy upstream code removed.

Also, remove functions and types used only by those two functions:

nxt_job_sockaddr_unix_parse()
nxt_job_sockaddr_inet6_parse()
nxt_job_sockaddr_inet_parse()
nxt_job_sockaddr_parse_t
nxt_job_resolve()
nxt_job_resolve_t

No functional changes.

Compared to the previous implementation based on OpenSSL, the new implementation
has these advantages:

 1. Strict and reliable detection of invalid strings, including strings with
    less than 4 bytes of garbage at the end;

 2. Allows to use Base64 strings without '=' padding.