commandserver: expand log path for convenience
This allows us to set the log path relative to $XDG_RUNTIME_DIR, for instance.
[cmdserver]
log = $XDG_RUNTIME_DIR/chg/server.log
commandserver: switch logging facility to ui.log() interface
The "pager subcommand" message is removed since ui isn't accessible there.
I think that's okay as cmdtable[cmd]() will call attachio() and some debug
message will be printed.