v0.2.1

CHANGELOG:

• Add new (Unix-only) set_fds_cloexec() helper
• FreeBSD: Don't use the kern.proc.nfds sysctl from close_open_fds() (since closefrom() can be used to provide a performance boost on the BSDs, and kern.proc.nfds may actually slow that down)
• Internal reorganization
• Improved documentation