add basic download callback

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

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

ALPM already makes it easy enough to get an error string; no point in
using a wrapper.

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

* add operation lookup function
* update existing line instead of writing a new line each time

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

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