security/samhain: housekeeping

• The port comes in three flavors, standalone, -server, and -client. All three use one or more files under /var/lib. Remove the logic that only includes that dir in plist for some of those flavors, and include it for all.
• Get rid of .sh extensions on rc scripts.
• Use INSTALL_SCRIPT instead of CP to ensure the rc scripts have useful permissions.
• Patch the rc script source so they properly respect settings in rc.conf.
• Use @postunexec in plist instead of the deprecated @unexec.

PR: 249433
Submitted by: Greg Veldman <freebsd@gregv.net> (maintainer)