Previously, these bits were ignored by Chmod, which sent the numerical
value of the mode argument as-is to the server. As a result, callers had
to supply POSIX values for setuid, setgid and sticky to Chmod.
The new version supports both the POSIX values and the Go values.
Also added a note to the docs to clarify that the umask is not
subtracted from the mode, and why that is. The only portable way to get
the umask is to set it, then reset it, but that's racy. On Linux, we
could parse /proc/self/status, but that doesn't work portably and will
fail where /proc is not available (some Docker containers, notably).