Doc: prefer sysctl to /proc/sys in docs and comments.

sysctl is more portable than Linux's /proc/sys file tree, and
often easier to use too.  That's why most of our docs refer to
sysctl when talking about how to adjust kernel parameters.
Bring the few stragglers into line.

Discussion: https://postgr.es/m/361175.1661187463@sss.pgh.pa.us

doc/src/sgml/runtime.sgml

@@ -1277,11 +1277,12 @@ default:\
     <itemizedlist>
      <listitem>
       <para>
-      On <productname>Linux</productname>
-      <filename>/proc/sys/fs/file-max</filename> determines the
-      maximum number of open files that the kernel will support.  It can
-      be changed by writing a different number into the file or by
-      adding an assignment in <filename>/etc/sysctl.conf</filename>.
+      On <productname>Linux</productname> the kernel parameter
+      <varname>fs.file-max</varname> determines the maximum number of open
+      files that the kernel will support.  It can be changed with
+      <literal>sysctl -w fs.file-max=<replaceable>N</replaceable></literal>.
+      To make the setting persist across reboots, add an assignment
+      in <filename>/etc/sysctl.conf</filename>.
       The maximum limit of files per process is fixed at the time the
       kernel is compiled; see
       <filename>/usr/src/linux/Documentation/proc.txt</filename> for

src/backend/postmaster/postmaster.c

@@ -4909,7 +4909,7 @@ SubPostmasterMain(int argc, char *argv[])
 	 * If testing EXEC_BACKEND on Linux, you should run this as root before
 	 * starting the postmaster:
 	 *
-	 * echo 0 >/proc/sys/kernel/randomize_va_space
+	 * sysctl -w kernel.randomize_va_space=0
 	 *
 	 * This prevents using randomized stack and code addresses that cause the
 	 * child process's memory map to be different from the parent's, making it