Guard unsafe imports behind !appengine

[Hexcles]

"unsafe" (in this case, imported via "golang.org/x/sys/unix") cannot be
used on AppEngine, so we need to add the "!appengine" build constraint
to files that use it (namely a few terminal_check_*.go).

Fixes #947.

[Hexcles]

@dgsb what do you reckon?

[dgsb]

@Hexcles did you try my suggestion to only add the !appengine build constraint in the terminal_check_unix.go file ?
I would also like that you merge master in your branch so that you can benefit a full cross compilation for all target supported by go 1.12 compiler in travis.

[Hexcles]

@dgsb sorry this thread has been going to my personal email which I haven't been keeping close track of.

I tried your suggestion on both Linux and macOS, and it does work. Only terminal_check_unix.go needs to be modified. I thought terminal_check_bsd.go would also need to be modified when I deployed from macOS, but apparently that's not the case from my experiment. It seems like the Go AppEngine toolchain is more like a crossbuild setup targeting Unix/Linux.

I also rebased the change on top of the current master.

[Hexcles]

ping @dgsb