agent: Changed the exit-signal from numeric to textual

Since the numeric value of various signals differs between,
architectures, send a textual signal identifier in the "exit-signal"
field when a spawned process exits with a signal.

Fixes #777
Closes #848
Signed-off-by: Stef Walter <stefw@redhat.com>
 * Updated copyright header
remotectl: Add a new process tool

With an empty certificate command for now
common: Refactor the certificate loading/generation code

Put into its own file.
remotectl: Implement the remotectl certificate command

This creates a self-signed certificate if one is not available.
It prints out the path of the certificate that will be used.

Can specify the user and group that should own the certificate.
ws: Run cockpit-ws as non-root user and group

Add configure arguments to --with-cockpit-user=xxx which defaults
to root, and --with-cockpit-group=yyyy which defaults to no group
being explicitly set.

cockpit-ws is run as the cockpit-ws user/group. cockpit-session
is a setuid root program.