This patch implements the OpenMP pinned memory trait for Linux hosts. On 
other hosts and on devices the trait becomes a no-op (instead of being 
rejected).

The memory is locked via the mlock syscall, which is both the "correct" 
way to do it on Linux, and a problem because the default ulimit for 
pinned memory is very small (and most users don't have permission to 
increase it (much?)). Therefore the code emits a non-fatal warning 
message if locking fails.

Another approach might be to use cudaHostAlloc to allocate the memory in 
the first place, which bypasses the ulimit somehow, but this would not 
help non-NVidia users.

The tests work on Linux and will xfail on other hosts; neither libgomp 
nor the test knows how to allocate or query pinned memory elsewhere.

The patch applies on top of the text of my previously submitted patches, 
but does not actually depend on the functionality of those patches.

OK for stage 1?

I'll commit a backport to OG11 shortly.

Andrew