Fix logging long named command lines

The saved_command_line buffer in bin/named/main.c is 8192 bytes long. The size of libisc's internal logging buffer (defined by the value of the LOG_BUFFER_SIZE constant in lib/isc/log.c) is also 8192 bytes. Since the buffer containing the ellipsis is passed as the last argument to isc_log_write() and the buffer containing the potentially trimmed named command line (saved_command_line) is passed as the second argument in the same isc_log_write() call, it may happen that saved_command_line will exhaust all available space in libisc's internal logging buffer, in which case the ellipsis will be elided from the output.

Make saved_command_line 4096 bytes long as that value is arguably also large enough for any reasonable use case and at the same time it ensures ellipsis will always be printed for excessively long named command lines.

Closes #1430 (closed)