Improved and portable ast_log recursion avoidance

This introduces a new logger routine ast_log_safe.  This routine should be
used for all error messages in code that can be run as a result of ast_log.
ast_log_safe does nothing if run recursively.  All error logging in
astobj2.c, strings.c and utils.h have been switched to ast_log_safe.

This required adding support for raw threadstorage.  This provides direct
access to the void* pointer in threadstorage.  In ast_log_safe, NULL is used
to signify that this thread is not already running ast_log_safe, (void*)1 when
it is already running.  This was done since it's critical that ast_log_safe
do nothing that could log during recursion checking.

ASTERISK-24155 #close
Reported by: Timo Teräs
Review: https://reviewboard.asterisk.org/r/4502/
........

Merged revisions 433522 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@433523 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/hashtab.c

@@ -43,10 +43,6 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 #include "asterisk/hashtab.h"
 
 
-#ifndef __AST_DEBUG_MALLOC
-void *_ast_mem_backtrace_buffer[_AST_MEM_BACKTRACE_BUFLEN];
-#endif
-
 #if (defined(MALLOC_DEBUG) && !defined(STANDALONE))
 static void _ast_hashtab_resize(struct ast_hashtab *tab, const char *file, int lineno, const char *func);
 #define ast_hashtab_resize(a)	_ast_hashtab_resize(a,__FILE__, __LINE__, __PRETTY_FUNCTION__)