Return EINVAL for O_SYMLINK on non-symlink

[WIP] Symlinks

Allow seek beyond end of file.

This is enough to fix as. I don't know if it correctly fills the space
with zeros when writing beyond the current EOF.

Use ? instead of try!

I didn't notice there were two copies of this code

Initialize fields of Stat struct to 0

Prevents potential undefined behavior based on how the call is made. Of
course, eventually these fields should actually be set.

fix travis macos build

trying to fix travis macos build fail. don't know if this work or not. I don't have mac to test it :)

Small formatting and documentation fixes

Merge fuse and scheme, add mkfs utility, remove old utility

Travis: fix config (on Linux)

With this change, travis will work on Linux at least.