kqueue

• rework internals #43
• add low-level functions
• only need to store flags on directories
• less mutexes #13
• done can be an unbuffered channel
• remove calls to os.NewSyscallError
• fix regression in rework causing subdirectories to be watched #48
• cleanup internal watch before sending remove event #51

other

• More efficient string concatenation for Event.String() #52 (thanks @mdlayher)