plugins/elements/gstfilesrc.*: Add "sequential" property, off by default, to use madvise and hint to the kernel that ...

Original commit message from CVS:
* 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_map_region):
* plugins/elements/gstfilesrc.h:
Add "sequential" property, off by default, to use madvise and hint
to the kernel that sequential access is desired.
Touch all retrieved pages by default to ensure they are pulled
into memory. (Closes #345720)