Initialise some flags in gs2 save and improve restart file treatment

• Make sure save_many and read_many are always initialised

• Accept any length string in call to set_restart_file

• Truncate set_restart_file argument if required (and warn)

Small tweaks to make it easier to use the gs2_save module without going through the full initialisation of gs2. In 8.2 we should probably seek to avoid run_name_size and instead adopt allocatable strings.

Prior to this PR, calling set_restart_file(some_short_string) would lead to run time memory troubles as the argument was not exactly run_name_size length.