2024-11-29-10efd31: #2688: Print available frontends and backends list when running synlig --help

Fixes: #2669

Fragment of help message:

    -L logfile
        like -l but open log file in line buffered mode

    -o outfile
        write the design to the specified file on exit

    -b backend
        use this backend for the output file specified on the command line
        list of available backends: aiger, blif, btor, cxxrtl, edif, firrtl,
        ilang, intersynth, jny, json, rtlil, simplec, smt2, smv, spice, table,
        test_autotb, verilog, xaiger

    -f frontend
        use the specified frontend for the input files on the command line
        list of available frontends: aiger, blif, ilang, json, liberty, rtlil,
        systemverilog, uhdm, verilog, verilog_with_uhdm, write_file, xaiger2

    -H
        print the command list

    -h command
        print the help message for the specified command

    -s scriptfile
        execute the commands in the script file

    -c tcl_scriptfile
        execute the commands in the tcl script file (see 'help tcl' for details)

Message on frontend not found error:

list of available frontends: aiger, blif, ilang, json, liberty, rtlil,
systemverilog, uhdm, verilog, verilog_with_uhdm, write_file, xaiger2

ERROR: No such frontend: xxx

Message on backend not found error:

list of available backends: aiger, blif, btor, cxxrtl, edif, firrtl, ilang,
intersynth, jny, json, rtlil, simplec, smt2, smv, spice, table, test_autotb,
verilog, xaiger

ERROR: No such backend: xxx