net: shell: Make shell commands non-static to allow reuse.

Some applications may want to reuse implementations of these
commands for debugging/diagnostics purpose even if they don't
use net shell per se, or implement an alternative shell.

Jira: ZEP-2064

Change-Id: I48cb66ccc41bd41a75a4eb8eb3c366316ec5a096
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>