move the dynamic string support in a better place i.e. string.h

While doing this, add a bit of documentation, and slightly
extend the functionality as follows:
 + a max_len of -1 means that we take whatever the current size
   is, and never try to extend the buffer;
 + add support for alloca()-ted dynamic strings, which is very
   useful for all cases where we do an ast_build_string() now.

Next step is to simplify the interface by using shorter names
(e.g. ast_str as a prefix) and removing the _thread variant
of the functions by saving the threadstorage reference into
the struct ast_str. This can be done by overloading the
'type' field.

Finally, I will do my best to remove the convoluted interface
that results from trying to support platforms without va_copy().



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@48509 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/utils.h

@@ -32,10 +32,10 @@
 #include <arpa/inet.h>	/* we want to override inet_ntoa */
 #include <netdb.h>
 #include <limits.h>
+#include <string.h>
 
 #include "asterisk/lock.h"
 #include "asterisk/time.h"
-#include "asterisk/strings.h"
 #include "asterisk/logger.h"
 #include "asterisk/compiler.h"
 
@@ -540,4 +540,5 @@ int _ast_vasprintf(char **ret, const char *file, int lineno, const char *func, c
  */
 void ast_enable_packet_fragmentation(int sock);
 
+#include "asterisk/strings.h"
 #endif /* _ASTERISK_UTILS_H */