sysutils/py-salt: add new rc scripts, clean up current scripts

• Add salt-proxy.in and associated pkg-message text for Salt Proxy
• Restore installation of salt-api.in accidentally removed in r396791
• Set a default for PATH to address upstream issue 27151
• While here, make headers more verbose and trim variable quotes per rclint

PR: 203262
Submitted by: Christer Edwards <christer.edwards@gmail.com> (maintainer)