cli: Created context module

Will hold various context variables.

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
cli: Add option to choose API address

Added API address in context module and added option to set it through
environment variable (OSV_API) or command line option [-a | --api]

Precendence:

* Command line argument: -a, --api
* Environment variable: OSV_API
* Default: http://127.0.0.1:8000

Closes #436.

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
implement reboot call

This is a simple wrapper around our reboot/halt/poweroff, and one that
Lyor has been requesting for a while.

Note, however, that this is the *glibc* version, not the Linux version.
The man page says:

  "Under glibc some of the constants involved have gotten
   symbolic names RB_*, and the library call is a 1-argument
   wrapper around the 3-argument system call:"

The constants have the same value and can be reused directly, but the
signature is that of the 1-argument glibc call.

Fixes #441

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>