Support symlinked drop-in configuration files

This MR teaches mkinitcpio to read drop-in configuration snippets from symlinks in the /etc/mkinitcpio.conf.d and /etc/cmdline.d directories. It does so by relaxing the find command to follow symlinks when checking their type, replacing the -type test with an -xtype test.

Motivation

I like to keep my configuration files in a central stow directory and then symlink them to the appropriate places. This allows me to check my system config into version control, like etckeeper enables, while maintaining control over the commits and git log.

For this to work, programs need to be symlink-aware, hence this MR.