Removed posix_spawn signal masks - they cannot be supported in NuttX; Add...

Removed posix_spawn signal masks - they cannot be supported in NuttX; Add skeleton for posix_spawn proposer - still incomplete

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@5503 42af7a65-404d-4744-a932-0658087f49c3