[oop] Fix OOP dbus service registration for numeric profile names

A purely-numeric profile name (such as Buteo automatically generates
for SyncML over Bluetooth profiles) cannot be used as a dbus service
path.  This commit improves the plugin_main.cpp code to prefix any
purely-numeric profile name with "profile-" before attempting to
register as a dbus service.
[buteo-syncfw] Fix OOPP server plugins

This commit fixes support for out-of-process server plugins.