Add __vsyslog_internal, with same flags as __v*printf_internal.

__nldbl___vsyslog_chk will ultimately want to pass PRINTF_LDBL_IS_DBL
down to __vfprintf_internal *as well as* possibly setting PRINTF_FORTIFY.
To make that possible, we need a __vsyslog_internal that takes the
same flags as printf.  The code in misc/syslog.c does also get a
little simpler.

Tested for powerpc and powerpc64le.