plugins/elements/: Add a use-mmap property to enable easier testing of all code paths.

plugins/elements/: Add a use-mmap property to enable easier testing of all code paths.

Original commit message from CVS:
* plugins/elements/gstelements.c:
* plugins/elements/gstfilesrc.c: (gst_file_src_class_init),
(gst_file_src_init), (gst_file_src_set_property),
(gst_file_src_get_property), (gst_file_src_start):
* plugins/elements/gstfilesrc.h:
Add a use-mmap property to enable easier testing of all code paths.
Bump rank to PRIMARY, so filesrc is the preferred file reader and used
in the absence of gnomevfssrc. (Closes #340501)