File based SessionHandler: Fix unlikely data corruption due to file creation race

There is a small time window between is_readable() returns true
and another process creates the file at the same time.
   -> we would truncate the data of the other process.

Fix it by using 'c+' fopen() mode which is available since PHP 5.2.6.
(Minimum PHP version for Horde is 5.3.0)

For those interested in the details of the PHP open modes, look for the
    PHPAPI int php_stream_parse_fopen_modes(const char *mode, int *open_flags)
function in the PHP source code.

Signed-off-by: Michael M Slusarz <slusarz@horde.org>

Bug #12557
[mms] Fix rare race condition when creating file-based session data for the first time (Thomas Jarosch <thomas.jarosch@intra2net.com; Bug #12557).