Use va_copy for stringfields

The ast_string_field_build_va functions were written to take to separate
va_lists to work around FreeBSD 4 not having va_copy defined.

In the end, we don't support anything using gcc < 3 anyway because we use
va_copy all over the place anyway. This patch just simplifies things by
removing the second va_list function arguments in favor of va_copy.

Review: https://reviewboard.asterisk.org/r/1233/
--This line, and those below, will be ignored--

M    include/asterisk/stringfields.h
M    main/utils.c
M    main/channel.c


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@320946 65c4cc65-6c06-0410-ace0-fbb531ad65f3