xl: return(N) from individual commands to top level instead of exit(N)

xl: return(N) from individual commands to top level instead of exit(N)

This allows the top level command dispatcher to cleanup some of its
own allocations.

This is a fairly mechanical conversion of exit(FOO) into return FOO
for the top-level command functions (i.e. main_*). There are still
code paths which will exit() further down the call chains which will
require actual thought.

At first glance not all the return codes are which one would normally
expect for process exit codes (e.g. some are negative) but I didn't
attempt to address that here.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>