session-func.c (set_option): Use symbols for `log-verbosity'.

* src/session-func.c (set_option)[SSH_OPTIONS_LOG_VERBOSITY]: Use
  symbols to represent log levels instead of numbers.  All callers
  updated.
  (set_sym_opt): New procedure.
* examples/echo/client.scm (main): Set log verbosity to `nolog'.
* examples/sssh.scm (main): Update.
* NEWS: Update.