Show enable_trezor in the version string if we were compiled
with `-tags enable_trezor`. And hide the `-trezor` flag from
the help output if we were not.
The trezor libraries are not yet stable enough to build
gocryptfs with trezor support by default.
It does not even compile at the moment:
$ ./build.bash -tags enable_trezor
# github.com/conejoninja/tesoro/vendor/github.com/trezor/usbhid
../../conejoninja/tesoro/vendor/github.com/trezor/usbhid/hid.go:32:11: fatal error: os/threads_posix.c: No such file or directory
#include "os/threads_posix.c"
^~~~~~~~~~~~~~~~~~~~
compilation terminated.
https://github.com/conejoninja/tesoro/issues/9
