Simplify beginning of database backup

It was unclear what was being errored out at the beginning of the
process. But it happens that it is just necessary to check if the
backup running is only an archive or not, then return a NULL file
list before continuing process. This should be part of some safety
checks though.
Simplify code for recovery target identification

Some unnecessary comments are removed and cleaned as well at the same
time.
Simplify code related to HAVE_DATABASE, HAVE_ARCLOG, TOTAL_READ_SIZE

Those macros were mainly used in code paths where they didn't make that
much sense, complicating heavily the code. Correct at the same time some
code comments.