libnfs.c: Support use of O_TRUNC with nfs_create

Simplify usage for application programmers by supporting the use of
O_TRUNC with nfs_create.

Signed-off-by: Ross Lagerwall <rosslagerwall@gmail.com>
libnfs.c: Handle OOM condition

Signed-off-by: Ross Lagerwall <rosslagerwall@gmail.com>
libnfs.h: Update open() flags docs

Signed-off-by: Ross Lagerwall <rosslagerwall@gmail.com>
Merge pull request #89 from rosslagerwall/create-trunc

Support use of O_TRUNC with nfs_create