Read arc aliases from all available configuration files

Summary: Fixes T10431. Updates the getAliases() method to read every arc configuration file.

Test Plan:
Added an arc duck alias to /etc/arcconfig, ran arc duck, saw "quack".

Added an arc pig alias to ~/.arcrc via arc alias, ran arc pig, saw "oink oink".

Reviewers: nevogd, chad, Blessed Reviewers

Reviewed By: chad, Blessed Reviewers

Subscribers: eadler, epriestley

Tags: #twitter

Maniphest Tasks: T10431

Differential Revision: https://secure.phabricator.com/D15342