Mark some functions in core.sys.posix.unistd as safe and nothrow
These functions should be trusted, not safe
Merge pull request #755 from tom-tan/safe-nothrow-core-sys-posix-unistd

Mark some functions in core.sys.posix.unistd as safe and nothrow
because calling toString() is impure, can allocate memory and throw
Merge pull request #756 from WalterBright/xmmflag

because calling toString() in stdarg is slow, impure, can allocate memory and throw