Draft: add slapd runscript

Add runscript for 'slapd' OpenLDAP standalone server in Debian.

$ wc /usr/share/runit/sv/slapd/run /etc/init.d/slapd 
  63  249 1655 /usr/share/runit/sv/slapd/run
 202  789 5435 /etc/init.d/slapd

Limitations compared with initscript:

• does not replicate compatibility logic "for OpenLDAP 2.1 client libraries"
• may not pass KRB5_KTNAME environment variable if specified in defaults (TBC)