prosody: fix build, bump to to 0.12.6 and add test.sh

prosody: fix build, bump to to 0.12.6 and add test.sh

mod_s2s was refactored into a single mod_s2s.lua in the flat modules
directory in 0.12.x; remove the obsolete subdirectory install rule that
caused the package build to fail.

Also add util/human/ and util/prosodyctl/ which are new subdirectories
in 0.12.x not covered by the previous install rules.

Force libidn instead of ICU for stringprep

prosody's configure script auto-detects stringprep backends and prefers
ICU when available, generating -DUSE_STRINGPREP_ICU and including
<unicode/usprep.h>. The OpenWrt SDK staging directory does not provide
ICU development headers, so the build fails with:

  encodings.c:271:10: fatal error: unicode/usprep.h: No such file or directory

The Makefile DEPENDS already declares +libidn. Pass --with-idn=idn to
explicitly select the libidn backend, which is available in the staging
directory.

Also, pass TARGET="../util/" to MAKE_FLAGS. OpenWrt has it's own TARGET
env var which clobbers the 'TARGET' var from prosody's build (specified
as 'TARGET?=../util/')

Adding a test.sh to check for the correct version (since prosody mostly
has lua scripts).

Security release addressing multiple vulnerabilities. 0.12.6 is
likely the last release of the 0.12.x series (EOL June 2026).

Upstream advisory: https://prosody.im/security/

Signed-off-by: Alexandru Ardelean <redacted>