ksmserver: Allow screen locker to be disabled with a config setting

The "--no-lockscreen" command line option is already present to disable screen locking if a third party screen locker is in use. However, this is an inconvenient setting to change, requiring a distro-installed file to be edited (the format and location of which may vary depending on whether systemd is in use). Having to make changes in that way risks either them being overwritten on an upgrade (if the original file is modified), or becoming obsolete and not receiving upgrades (if a site or user copy is substituted). A site change will also apply to all users regardless of their personal preference.

This change adds a config key "[General]noScreenLocker" to ksmserverrc, which if it is set to "true" has the same effect as the command line option, but does not override the command line option if one is specified. This allows individual users who wish to use a third party screen locker to have a personal configuration, without having to edit any distro-provided files.