Protect the #define MALLOC_PRODUCTION directive in case it was also
defined on the command line (due to MALLOC_PRODUCTION=yes in src.conf).
MFC r240774:
Don't use global nfs_root_node variable as per-file storage. There are
fields that should be file-specific.
MFC  r240780, r252468:
Make nfs_readdir() more careful about using response data, cached in global
buffer. For now it fixes bug when following `ls` command will return data
from previous one aborted by pager. Also it should allow to read several
directories same time, for example, for recursive tracerse.