OSHMEM: Fix mmap compatibility issue

Using MAP_ANONYMOUS brings different interpretation of fd, offset arguuments of mmap()
Linux:
The mapping is not backed by any file; the fd and offset arguments are ignored.
Mac:
Map anonymous memory not associated with any specific file.  The offset argument is
ignored.  Mac OS X specific: the file descriptor used for creating MAP_ANON regions
can be used to pass some Mach VM flags, and can be specified as -1 if no such flags
are associated with the region
FreeBSD:
Map anonymous memory not associated with any specific file.  The file
descriptor used for creating MAP_ANON must be -1.  The offset   argument must be 0.

fixed by Igor, reviewed by Mike

Refs #4399

cmr=v1.7.5:reviewer=ompi-rm1.7