Handling of build-time enabled GUC settings

I've noticed some inconsistencies in the handling of build-time enabled GUC 
setttings.  For example, a non-SSL enabled server will happily accept ssl = 
on and simply ignore it.  On the other hand, several syslog related 
parameters are not accepted at all when syslog is not enabled.

I propose that in the interest of greatest flexibility, configuration-file 
sharing etc. we do it as follows: All parameters are accepted all the time.  
(The exception will be the few odd debugging parameters which are marked 
GUC_NOT_IN_SAMPLE; we can handle those any way we want to.)  Those that 
cannot take effect because of build-time configuration have to use a 
check/assign hook to prevent settings that cannot be honored.  This would 
mean that ssl = on has to be prevented.  As another example, syslog_facility 
and syslog_ident are accepted with all values but log_destination will not 
accept syslog if that is not available.  (The latter is the current 
behavior.)