sysutils/burp-devel: Update to 2.3.24

• Protocol 2: warn and skip on verify/restore of unsupported file types.
• Resurrect, improve and use sysutils/burp/files/burp.in instead of the rc

script from distfile, both for sysutils/burp (master port) and for
sysutils/burp-devel (slave port): this is necessary because, while
sysutils/burp's distfile still distributes the script, sysutils/burp-devel's
distfile does not anymore starting with version 2.3.24. I also made a few
modifications to the file so that it behaves well with non standard PREFIX
values.

Reviewed by: 0mp, gerald, dbaio, adamw
Approved by: tcberner (co-mentor)
Differential Revision: https://reviews.freebsd.org/D24104