doc: Mention more variant --name=value of -c name=value for postgres

postgres --name=value and -c name=value are equivalents.  This commit
expands the documentation of libpq's "option" connection parameter and
the server startup sequence for shell interactions to mention both
rather than only -c.

Extracted from a larger patch by the same authors.

Reported-by: Alexey Palazhchenko
Author: David Johnston, Aleksander Alekseev
Reviewed-by: Nathan Bossart, Peter Eisentraut, Álvaro Herrera
Discussion: https://postgr.es/m/CAJ7c6TMkuLiLfrA+EFCPYfhXoMKRxxssB5c86+ibxfaz6+=Sdg@mail.gmail.com

doc/src/sgml/config.sgml

@@ -334,9 +334,10 @@ UPDATE pg_settings SET setting = reset_val WHERE name = 'configuration_parameter
       <para>
        During server startup, parameter settings can be
        passed to the <command>postgres</command> command via the
-       <option>-c</option> command-line parameter.  For example,
+       <option>-c name=value</option> command-line parameter, or its equivalent
+       <option>--name=value</option> variation.  For example,
 <programlisting>
-postgres -c log_connections=yes -c log_destination='syslog'
+postgres -c log_connections=yes --log-destination='syslog'
 </programlisting>
        Settings provided in this way override those set via
        <filename>postgresql.conf</filename> or <command>ALTER SYSTEM</command>,
@@ -353,10 +354,10 @@ postgres -c log_connections=yes -c log_destination='syslog'
       of the session, but do not affect other sessions.
       For historical reasons, the format of <envar>PGOPTIONS</envar> is
       similar to that used when launching the <command>postgres</command>
-      command; specifically, the <option>-c</option> flag must be specified.
-      For example,
+      command; specifically, the <option>-c</option>, or prepended
+      <literal>--</literal>, before the name must be specified. For example,
 <programlisting>
-env PGOPTIONS="-c geqo=off -c statement_timeout=5min" psql
+env PGOPTIONS="-c geqo=off --statement-timeout=5min" psql
 </programlisting>
      </para>
 

doc/src/sgml/libpq.sgml

@@ -1421,9 +1421,10 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
       <listitem>
        <para>
         Specifies command-line options to send to the server at connection
-        start.  For example, setting this to <literal>-c geqo=off</literal> sets the
-        session's value of the <varname>geqo</varname> parameter to
-        <literal>off</literal>.  Spaces within this string are considered to
+        start.  For example, setting this to <literal>-c geqo=off</literal>
+        or <literal>--geqo=off</literal> sets the session's value of the
+        <varname>geqo</varname> parameter to <literal>off</literal>.
+        Spaces within this string are considered to
         separate command-line arguments, unless escaped with a backslash
         (<literal>\</literal>); write <literal>\\</literal> to represent a literal
         backslash.  For a detailed discussion of the available