Let old-style (shell-based) jail configuration handle jail names that
contain characters not allowed in a shell variable (such as "-").
These will be replaced by an underscore in jail config variables,
e.g. for jail "foo-bar" you would set "jail_foo_bar_hostname".

This is separate from the current code that changes the jail names
if they contain "." or "/".  It also doesn't apply to jails defined
in a jail.conf file.