build stuff:
 - fix githead_from_path.sh to not use bashisms
 - make it return a slightly longer string so that cmake substring
   for git abbrev will not fail
 - add libntirpc github repo url to error message when submodule not
   here, for people who build without git
 - automatically disable nfsidmap if not found like most other
   features

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Log: Add a Log_FreeThreadContext function and use it in 9P dispatcher threads

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
9P/RDMA: use a send buffer queue instead of the 1-1 matching with recv ones

This saves 32MB/client in the default config at the cost of a negligible performance hit.

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
9P/RDMA: fix new output buffer queue with multiple IB adapters

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
9P: create dispatcher threads with PTHREAD_CREATE_DETACHED since they're not joined

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>