agent: Changed exit-signal from numeric to textual.

Changed exit-signal of on_pipe_close in src/agent/cockpittextstream.c from numeric to textual identifier.
Added the implementation of cockpit_strsignal and related test cases.
Updated modules/shell/test-spawn.html.
License information needs to be updated.

Fixes #777
This should just use g_strdup(), rather than g_strdup_printf(). In addition the braces surrounding this line can be removed.