src/server-func.c (set_option): Use symbols for `log-verbosity'.

* src/server-func.c (set_option) [SSH_BIND_OPTIONS_LOG_VERBOSITY]:
  Use symbols to represent log levels instead of numbers.  All
  callers updated.
  (set_sym_opt): New procedure.
* src/session-func.c: Move `log_verbosity' to `common.c' file.
* src/common.c, src/common.h: Update.
* examples/echo/server.scm (*default-log-verbosity*): Set to 'nolog.
* examples/ssshd.scm (*default-log-verbosity*): Likewise.
* NEWS: Update.