lsu: Add helper to merge config file options, convert server.

lsu: Add helper to merge config file options, convert server.

After the command line options have been parsed, most paraslash
executables read options from a config file. The two lopsub parse
result structures are then merged in a way that command line options
take preference over config file options. This logic is duplicated
in all executables.

This patch introduces a generic helper to eliminate the duplication.
The new lsu_merge_config_file_options() will eventually be employed
by all executables which need to parse the config file. This patch,
however, only converts para_server.