Adds a SelectByName method to the archive and scanner which only require
the filename as input, and can thus be run before calling lstat on the
file. Can speed up scanning significantly if a lot of filename excludes
are used.
This patch should fix the following panic when trying to backup the
root filesystem with thre --one-file-system flag:
% restic backup --one-file-system /
(...)
panic: item /, device id 2082 not found, allowedDevs: map[/:2082]
