[PROPOSAL] libsvn-fs-util: Move more macros from fsfs and bdb

Hello,

There is a duplicate definition of the macro NOT_FOUND() in fsfs and bdb
implementations. I wish to move this to libsvn-fs-util library.

On the other hand, the macro ALREADY_EXISTS(), to check if the file
already exists in the transaction, and the macro NOT_TXN(), to check if
it is a valid transaction root are defined for bdb, while they are
defined as functions for fsfs. I wish to move them as macros to
libsvn-fs-util library and use them in both fsfs and bdb implementation.

But, I'm not sure if it is relevant to make this change. Comments are
welcome. Thank you.

-- 
Regards,
Bhuvaneswaran