defaults-group-suffix in print_defaults

Also clarify which --{no-,}default* options, must be first.

Sample output:
$  client/mysql --help
client/mysql  Ver 15.1 Distrib 5.5.59-MariaDB, for Linux (x86_64) using readline 5.1
Copyright (c) 2000, 2017, Oracle, MariaDB Corporation Ab and others.

Usage: client/mysql [OPTIONS] [database]

Default options are read from the following files in the given order:
/etc/my.cnf /etc/mysql/my.cnf ~/.my.cnf
The following groups are read: mysql client client-server client-mariadb
The following options may be given as the first argument:
--print-defaults          Print the program argument list and exit.
--no-defaults             Don't read default options from any option file.
The following specify which files/groups are read (specified before other options):
--defaults-file=#         Only read default options from the given file #.
--defaults-extra-file=#   Read this file after the global files are read.
--defaults-group-suffix=# Additionally read default groups with # appended as a suffix.

tests running from build directory:
TEST: print defaults ignored as not first
$ sql/mysqld  --no-defaults --print-defaults --lc-messages-dir=${PWD}/sql/share

TEST: no startup occurs as --print-defaults specified
$ sql/mysqld  --print-defaults --lc-messages-dir=${PWD}/sql/share
sql/mysqld would have been started with the following arguments:
--lc-messages-dir=/home/dan/repos/build-mariadb-5.5/sql/share

TEST: default args can't be anywhere
$ client/mysql --user=bob  --defaults-file=/etc/my.cnf
client/mysql: unknown variable 'defaults-file=/etc/my.cnf'
$ client/mysql --user=bob  --defaults-group-suffix=.group
client/mysql: unknown variable 'defaults-group-suffix=.group'

/etc/my.cnf:
[client-server.group]
socket=/var/lib/mysql-multi/group/mysqld.sock
user=bob

/etc/my.other.cnf:
socket=/var/lib/mysql-other/mysqld.sock

TEST: defaults file read and suffix also applied
$  client/mysql  --defaults-file=/etc/my.other.cnf  --defaults-group-suffix=.group
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/lib/mysql-other/mysqld.sock' (2)

TEST: defaults extra file
$ client/mysql  --defaults-extra-file=/etc/my.other.cnf  --defaults-group-suffix=.group
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/lib/mysql-other/mysqld.sock' (2)

mysql-test/r/mysqld--help.result

 The following options may be given as the first argument:
 --print-defaults Print the program argument list and exit.
 --no-defaults Don't read default options from any option file.
+The following specify which files/extra groups are read (specified before remaining options):
 --defaults-file=# Only read default options from the given file #.
 --defaults-extra-file=# Read this file after the global files are read.
+--defaults-group-suffix=# Additionally read default groups with # appended as a suffix.
 
  --allow-suspicious-udfs 
  Allows use of UDFs consisting of only one symbol xxx()

mysys/default.c

@@ -1102,10 +1102,12 @@ void print_defaults(const char *conf_file, const char **groups)
     }
   }
   puts("\nThe following options may be given as the first argument:\n\
---print-defaults        Print the program argument list and exit.\n\
---no-defaults           Don't read default options from any option file.\n\
---defaults-file=#       Only read default options from the given file #.\n\
---defaults-extra-file=# Read this file after the global files are read.");
+--print-defaults          Print the program argument list and exit.\n\
+--no-defaults             Don't read default options from any option file.\n\
+The following specify which files/extra groups are read (specified before remaining options):\n\
+--defaults-file=#         Only read default options from the given file #.\n\
+--defaults-extra-file=#   Read this file after the global files are read.\n\
+--defaults-group-suffix=# Additionally read default groups with # appended as a suffix.");
 }