libalpm: add iterator interface for syncdb files

This commit adds an iterator interface for reading files from the
syncdbs. Instead of using alpm_pkg_get_files(), you now get the files
from the database using alpm_db_files_open(), you then use
alpm_db_files_next() to iterate through the files for each package. If
you want to actually load the files from that package you then use
alpm_db_files_load().

This means alpm_pkg_get_files() will always return empty for syncdbs,
even on .files databases, however these functions still work on the
localdb and loaded packages.

This aproach is faster when dumping the entire file list but slower when
searching for a specific package.

The memory usage of pacman is drastically less. See below.

build/pacman -Fl        0.55s user 0.01s system 99% cpu 0.556 total
build/pacman -Fl pacman 0.46s user 0.01s system 99% cpu 0.472 total
build/pacman -Fx pacman 2.88s user 0.09s system 99% cpu 2.965 total

pacman -Fl              1.60s user 0.13s system 99% cpu 1.731 total
pacman -Fl pacman       0.24s user 0.04s system 99% cpu 0.283 total
pacman -Fx pacman       2.45s user 0.14s system 99% cpu 2.593 total

                         Peak Memory
build/pacman -Fl         43.52MB
build/pacman -Fl pacmam  11.292MB

pacman -Fl               677.048MB
pacman -Fl pacman        163.288MB
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