* The design is similar to bash history:
* The CLI loads/saves its complete history to a file on entry and exit, respectively
* The size (number of lines) of the file is the same as the history in memory
* Only the latest session dumping its history will survive (bash merges multiple session history).
* Tilde-expansion is supported
* Files not found or without appropriate access will not cause an exit but will be logged at debug level
* New config options: CLICON_CLI_HIST_FILE with default value `~/.clixon_cli_history`
* New config options: CLICON_CLI_HIST_SIZE with default value 300.
* Added clicon_socket_set() and clicon_socket_get() functions for cleaning up backend server and restconf FCGI socket on termination.
* Removed obsolete `-x` command-line option to clixon_cli.
