Split g_file_open() into _ro() and _rw() variants

Rename g_file_open() to g_file_open_rw(), and add a new g_file_open_ro()
call that wraps the common g_file_open_ex(file, 1, 0, 0, 0) idiom. This
will make the file access mode more explicit in the code.

Change all calls to g_file_open() to the _ro() or _rw() variant as
appropriate, and replace g_file_open_ex(file, 1, 0, 0, 0) with the _ro()
call.

Lastly, add tests for the two new calls to test_os_calls.c (code
courteously provided by matt335672).

keygen/keygen.c

@@ -367,7 +367,7 @@ save_all(const char *e_data, int e_len, const char *n_data, int n_len,
         }
     }
 
-    fd = g_file_open(filename);
+    fd = g_file_open_rw(filename);
 
     if (fd != -1)
     {