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).

common/os_calls.c

@@ -2086,24 +2086,14 @@ g_memcmp(const void *s1, const void *s2, int len)
 /*****************************************************************************/
 /* returns -1 on error, else return handle or file descriptor */
 int
-g_file_open(const char *file_name)
+g_file_open_rw(const char *file_name)
 {
 #if defined(_WIN32)
     return (int)CreateFileA(file_name, GENERIC_READ | GENERIC_WRITE,
                             FILE_SHARE_READ | FILE_SHARE_WRITE,
                             0, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, 0);
 #else
-    int rv;
-
-    rv =  open(file_name, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
-
-    if (rv == -1)
-    {
-        /* can't open read / write, try to open read only */
-        rv =  open(file_name, O_RDONLY);
-    }
-
-    return rv;
+    return open(file_name, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
 #endif
 }
 
@@ -2145,6 +2135,14 @@ g_file_open_ex(const char *file_name, int aread, int awrite,
 #endif
 }
 
+/*****************************************************************************/
+/* returns -1 on error, else return handle or file descriptor */
+int
+g_file_open_ro(const char *file_name)
+{
+    return g_file_open_ex(file_name, 1, 0, 0, 0);
+}
+
 /*****************************************************************************/
 /* returns error, always 0 */
 int