FSIniter godoc was stale and wrong
Document InitRequest, InitResponse MaxWrite behavior
Add consts for open flags, including O_ACCMODE, and provide helper functions

O_ACCMODE has been confusing to people, and only exported by syscall.
Export it here for ease of discovery.

As O_RDONLY etc tests are easy to get wrong, if you mistakenly think
they are 1-bit flags, provide convenience methods like IsReadOnly().

We choose to go with Go-style names for the constants, instead of
the original C-style O_FOO names.
Use OpenFlag constants internally
Use the Go naming for OpenFlags constants consistently