Allow some kind of substitution on registration file.

[PerryWerneck]

When using slpd on a large number of workstation it would be really helpfull if one can use some kind of macro (like ${localaddr}) on registry file allowing using the same file for all workstations, for example:

`
service:install.linux:http://${localaddr}/repo,en,65535

watch-port-tcp=80

description=Instalation repository
`