lang/sbcl: Install default system-wide startup script

• patch to search for the system-wide startup script under ${PREFIX}/etc/
• patch documentation, so it refers to ${PREFIX}/etc/ instead of /etc
• startup script loads asdf and sets asdf:*central-registry* to the location where the cl-* ports install asd files

Approved by: krion (maintainer)
Differential Revision: https://reviews.freebsd.org/D12018