[foreign-memaccess] RFR 8229181: Add support for mapped memory segments

[Maurizio Cimadamore]

Hi,
this patch adds support for 'mapped' memory segments - that is, memory 
segments backed by memory-mapped files.

Webrev:
http://cr.openjdk.java.net/~mcimadamore/panama/8229181/

The API is straightforward - it merely adds a new factory to 
MemorySegment which takes a Path, a size (in long) and a MapMode.

The implementation will create a FileChannel (with the correct inferred 
OpenOptions), and then map the file using the new 'mapInternal' routine. 
This will return an Unmapper object which can be used to query 
properties of the mapping, as well as to unmap the file.

I'd like to thank Steve Dohrmann (who proposed some of the changes in 
this patch), as well as Alan Bateteman for providing many useful 
insights on how to tweak the FileChannelImpl internals so that they 
could be reused from memory segments.

Safety-wise, creating multiple segment from same path should be handled 
correctlty (as nmap does the right thing here). However Alan pointed me 
at a possible source of unsafety where the underlying file gets 
truncated - in which case parts of the mapping becomes invalid - which 
can crash the VM (with a SIGBUS signal, on Linux). It seems though that, 
since all our memory access occurs via Unsafe, there are already signal 
handlers installed by the VM that are prepared to catch these conditions 
(e.g. access to an unmapped page) - I did a quick test and it seems like 
that, after a truncate(), memory access fails gracefully (with an 
exception, not a crash), so I think we should be good here.

Maurizio