[[ Cleanup ]] revxml: Use const C strings whereever appropriate.

Removes a large number of GCC "write-strings" warnings.
[[ Cleanup ]] revdb: Unsigned int is not a pointer type.
[[ Cleanup ]] revdb: Remove some unused variables.
[[ Cleanup ]] revdb: Remove some set-but-unused variables.
[[ Cleanup ]] revdb: computehash(): Remove unused function.
[[ Cleanup ]] revdb: _strrev(): Remove unused function.
[[ Cleanup ]] revdb: REVDB_DATABASETYPECOUTN is of type size_t.
[[ Cleanup ]] revdb: printf %.* expects an int argument.

On most 64-bit platforms, sizeof(ptrdiff_t) > sizeof(int).
[[ Cleanup ]] revdb: The connection ID is an unsigned integer.
[[ Cleanup ]] revdb: Use correct size_type for connection count.