pacman.8: document .pacorig files

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
add.c: refactor backup file modification checks

The previous implementation was overly complex with unnecessary checks
and nested conditionals.  By reordering the tests and changing them to
all be checks for positive hash matches rather than non-matches, we can
collapse several cases and make the process much more linear.  This
removes the need to set hash_orig = "" just to reach some of the checks
and corrects a faulty assumption that files are equivalent when the
hashing process fails.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>