dns: Add core DNS API + unit tests and res_resolver_unbound module + unit tests.

This change adds an abstracted core DNS API which resembles the API described
here[1]. The API provides a pluggable mechanism for resolvers and also a
consistent view for records. Both synchronous and asynchronous queries are
supported.

This change also adds a res_resolver_unbound module which uses the libunbound
library to provide resolution.

Unit tests have also been written for all of the above to confirm the API and
functionality.

ASTERISK-24834 #close
Reported by: Matt Jordan

ASTERISK-24836 #close
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/4474/
Review: https://reviewboard.asterisk.org/r/4512/

[1] https://wiki.asterisk.org/wiki/display/AST/Asterisk+DNS+API


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@433370 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configure.ac

@@ -507,6 +507,7 @@ AST_EXT_LIB_SETUP([TERMCAP], [Termcap], [termcap])
 AST_EXT_LIB_SETUP([TIMERFD], [timerfd], [timerfd])
 AST_EXT_LIB_SETUP([TINFO], [Term Info], [tinfo])
 AST_EXT_LIB_SETUP([TONEZONE], [tonezone], [tonezone])
+AST_EXT_LIB_SETUP([UNBOUND], [unbound], [unbound])
 AST_EXT_LIB_SETUP([UNIXODBC], [unixODBC], [unixodbc])
 AST_EXT_LIB_SETUP([VORBIS], [Vorbis], [vorbis])
 AST_EXT_LIB_SETUP([VPB], [Voicetronix API], [vpb])
@@ -2032,6 +2033,8 @@ AST_EXT_TOOL_CHECK([NETSNMP], [net-snmp-config], , [--agent-libs],
 
 AST_EXT_LIB_CHECK([NEWT], [newt], [newtBell], [newt.h])
 
+AST_EXT_LIB_CHECK([UNBOUND], [unbound], [ub_ctx_create], [unbound.h], [])
+
 AST_EXT_LIB_CHECK([UNIXODBC], [odbc], [SQLConnect], [sql.h], [])
 
 AST_EXT_LIB_CHECK([OGG], [ogg], [ogg_sync_init], [])