Consistently save errno immediately after the operation setting it

Prevent the situation where errno is set by function A, then function B
is called (which is typically _(), but could be anything else) and it
overwrites errno, then errno is checked by the caller.

errno is a horrific API, and we need to be careful to save its value as
soon as a function call (which might set it) returns. i.e. Follow the
pattern:
  int errsv, ret;
  ret = some_call_which_might_set_errno ();
  errsv = errno;

  if (ret < 0)
    puts (strerror (errsv));

This patch implements that pattern throughout GLib. There might be a few
places in the test code which still use errno directly. They should be
ported as necessary. It doesn’t modify all the call sites like this:
  if (some_call_which_might_set_errno () && errno == ESOMETHING)
since the refactoring involved is probably more harmful than beneficial
there. It does, however, refactor other call sites regardless of whether
they were originally buggy.

https://bugzilla.gnome.org/show_bug.cgi?id=785577

tests/timeloop-basic.c

@@ -25,7 +25,8 @@ my_pipe (int *fds)
 {
   if (pipe(fds) < 0)
     {
-      fprintf (stderr, "Cannot create pipe %s\n", strerror (errno));
+      int errsv = errno;
+      fprintf (stderr, "Cannot create pipe %s\n", strerror (errsv));
       exit (1);
     }
 }
@@ -121,7 +122,7 @@ input_callback (int source, int dest)
 void
 create_child (int pos)
 {
-  int pid;
+  int pid, errsv;
   int in_fds[2];
   int out_fds[2];
   
@@ -129,6 +130,7 @@ create_child (int pos)
   my_pipe (out_fds);
 
   pid = fork ();
+  errsv = errno;
 
   if (pid > 0)			/* Parent */
     {
@@ -150,7 +152,7 @@ create_child (int pos)
     }
   else				/* Error */
     {
-      fprintf (stderr,"Cannot fork: %s\n", strerror (errno));
+      fprintf (stderr,"Cannot fork: %s\n", strerror (errsv));
       exit (1);
     }
 }