Patch for current_schemas to optionally include implicit schemas

Following my discussion with Tom on -hackers a while ago about finding
the full schema search path, I decided to give it a go myself and
hopefully both learn something, and give Tom one less thing to do!

Attached is the result which seems to work OK here. The existing
current_schemas() works as previously, & I have added
current_schemas(bool) which will return the full implicit/explicit path
if passed true or the explicit path is pass false.

Thorough checking is advised in case I've done something nasty or missed
something I was unaware of (being my first venture into the backend
code) :-)

Regards, Dave.