Exclusive create support in container mode

add O_EXCL for create opeartion if necessary
check O_EXCL flag before O_TRUNC

issue #249.
Merge pull request #259 from ZhangZhenhua/o_excl

Exclusive create support in container mode
Update CMakeLists.txt and Changelog for tagging 2.4rc3.
Change the unit test tools to be compatible with plfs_chmod

The container directory need the execution bit so that PLFS can
access the files inside it.
Merge pull request #262 from zhang-jingwang/master

Update the unit test tools according to the new behavior of plfs_chmod in container mode

Small change in the tests subdir
Merge branch 'master' of https://github.com/plfs/plfs-core
Merge branch 'master' of https://github.com/plfs/plfs-core