Centralize logic for restoring errno in signal handlers.

Presently, we rely on each individual signal handler to save the
initial value of errno and then restore it before returning if
needed.  This is easily forgotten and, if missed, often goes
undetected for a long time.

In commit 3b00fdba9f, we introduced a wrapper signal handler
function that checks whether MyProcPid matches getpid().  This
commit moves the aforementioned errno restoration code from the
individual signal handlers to the new wrapper handler so that we no
longer need to worry about missing it.

Reviewed-by: Andres Freund, Noah Misch
Discussion: https://postgr.es/m/20231121212008.GA3742740%40nathanxps13

doc/src/sgml/sources.sgml

@@ -1007,18 +1007,10 @@ MemoryContextSwitchTo(MemoryContext context)
 static void
 handle_sighup(SIGNAL_ARGS)
 {
-    int         save_errno = errno;
-
     got_SIGHUP = true;
     SetLatch(MyLatch);
-
-    errno = save_errno;
 }
 </programlisting>
-     <varname>errno</varname> is saved and restored because
-     <function>SetLatch()</function> might change it. If that were not done
-     interrupted code that's currently inspecting <varname>errno</varname> might see the wrong
-     value.
     </para>
    </simplesect>