Followup to 7e94e41, add missing cache.[ch] files.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
Fix issue #30: Refactor DDNS service providers as plugins.

This commit moves all DDNS providers from src/ddns.c to plugins/*.c as
described in issue #30.  Many DDNS service providers share the same API
as DynDNS, these are therefore located in plugins/dyndns.c with slightly
different URLs only.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
Bump version to 1.99.7-beta2 and change dist formt bz2-->xz

Also, cleanup and add some useful stuff from finit and uftpd projects,
like automatic dependency rules and stripped installation.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>