Make bin/arc less linux-centered, more UNIX-agnostic
Make bin/arc less linux-centered, more UNIX-agnostic

Summary: Using /usr/bin/env in #! to allow the arc script to run as is in every UNIX OSes, like FreeBSD where bash is installed in /usr/local/bin/bash.

Test Plan: Tested on Solaris, Ubuntu, kFreeBSD/Debian and FreeBSD.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T1440

Differential Revision: https://secure.phabricator.com/D2913

chundongwang/arcanist,PageUpPeopleOrg/arcanist-patches,r4nt/arcanist,sneakpeeq/arcanist,jra3/arcanist,coursera/arcanist,r4nt/arcanist,Symplicity/arcanist,eSpark/arcanist,memsql/arcanist,fivestars/arcanist,folsom-labs/arcanist,haskell-infra/arcanist,aik099/arcanist,hach-que/arcanist,zhihu/arcanist,Symplicity/arcanist,memsql/arcanist,KieranHayes/arcanist,hach-que/arcanist,wharris/arcanist,eSpark/arcanist,wxstars/arcanist,phacility/arcanist,sneakpeeq/arcanist,haskell-infra/arcanist,wharris/arcanist,kevinseelbach/arcanist,dguo-coursera/arcanist,folsom-labs/arcanist,phacility/arcanist,eSpark/arcanist,haskell-infra/arcanist,codelabs-ch/arcanist,haskell-infra/arcanist,PageUpPeopleOrg/arcanist-patches,Khan/arcanist,kevinseelbach/arcanist,KieranHayes/arcanist,wikimedia/phabricator-arcanist,wharris/arcanist,PageUpPeopleOrg/arcanist-patches,julkiewicz/arcanist,Symplicity/arcanist,wxstars/arcanist,alexmv/arcanist,hach-que/arcanist,hach-que/arcanist,sneakpeeq/arcanist,coursera/arcanist,calblueprint/arcanist,alexmv/arcanist,calblueprint/arcanist,alexmv/arcanist,chundongwang/arcanist,zhihu/arcanist,alexmv/arcanist,uber/arcanist,phacility/arcanist,wxstars/arcanist,fivestars/arcanist,codelabs-ch/arcanist,kevinseelbach/arcanist,codelabs-ch/arcanist,dguo-coursera/arcanist,uber/arcanist,r4nt/arcanist,wikimedia/phabricator-arcanist,jra3/arcanist,r4nt/arcanist,uber/arcanist,dguo-coursera/arcanist,wikimedia/phabricator-arcanist,codelabs-ch/arcanist,fivestars/arcanist,Khan/arcanist,calblueprint/arcanist,codelabs-ch/arcanist,wxstars/arcanist,memsql/arcanist,Khan/arcanist,uber/arcanist,zhihu/arcanist,wharris/arcanist,KieranHayes/arcanist,PageUpPeopleOrg/arcanist-patches,coursera/arcanist,calblueprint/arcanist,folsom-labs/arcanist,fivestars/arcanist,Khan/arcanist,Symplicity/arcanist,chundongwang/arcanist,phacility/arcanist,jra3/arcanist,dguo-coursera/arcanist,eSpark/arcanist,jra3/arcanist,wikimedia/phabricator-arcanist,zhihu/arcanist,coursera/arcanist,sneakpeeq/arcanist,aik099/arcanist,KieranHayes/arcanist,julkiewicz/arcanist,phacility/arcanist,aik099/arcanist,chundongwang/arcanist,julkiewicz/arcanist,julkiewicz/arcanist,kevinseelbach/arcanist,memsql/arcanist,aik099/arcanist,folsom-labs/arcanist