sysutils/rsyslog8: fix build with libressl, add DTLS option

rsyslog8 fails to build with libressl due to lack of support of the
SSL_CONF_CTX type. This occurs in a block of code only used by the
DTLS input and output modules, which are not enabled by default in the
FreeBSD port.

Therefore solve the build failure by making the problematic function
dependant on LIBRESSL_VERSION_NUMBER being defined (via
files/patch-runtime_net__ossl.c, obtained from github issue 5342 for
rsyslog). This is similar to previous libressl compatability fixes
from PR 254864.

While here, add a DTLS option to enable building the imdtls and omdtls
modules -- this does require linkage against openssl to compile
successfully. Additionally required patches are to add the necessary
header files for the pthread_kill(3) and close(2) prototypes. I have
not been able to test the functionality of the DTLS modules very
effectively and would be glad to hear of anyone's experiences with
them.

PR: 278018
Reported by: Dave Hayes <dave@jetcafe.org>
Obtained from: (partly) https://github.com/rsyslog/rsyslog/issues/5342