gst/multifile/: Use g_file_[sg]et_contents() instead of using stdio functions.

gst/multifile/: Use g_file_[sg]et_contents() instead of using stdio functions.

Original commit message from CVS:
* gst/multifile/gstmultifilesink.c:
* gst/multifile/gstmultifilesrc.c:
Use g_file_[sg]et_contents() instead of using stdio functions.
Should be less error prone.
* tests/check/elements/multifile.c:
Create a temporary directory using standard functions instead of
creating a directory in the current dir.