lib: Add i_realloc_type() for i_realloc() that checks for overflows

lib: Simplify static checker's job for i_free() macro.

lib: [ip]_free(mem) is now also guaranteed to set mem=NULL It was already doing it, but updated the macros and comments to make it clear that this won't change in future. Changing this would only make it more likely that bugs occur and the performance improvements would be close to nonexistent.

lib: Use __attribute__((returns_nonnull)) for the common memory/string functions. Also added a couple of missing ATTR_MALLOCs.

Renamed __attr_*__ to ATTR_*. Renamed __attrs_used__ to ATTRS_DEFINED. --HG-- branch : HEAD

Changed .h ifdef/defines to use <NAME>_H format. --HG-- branch : HEAD

Added system_clean_pool. default_pool is now set statically, so it can be changed before calling lib_init(). Use malloc_usable_size() if it's available for asserts and for implementing system_clean_pool. --HG-- branch : HEAD

Use malloc attribute for the most commonly used memory and string allocation functions. Use warn_unused_result attribute for reallocs. --HG-- branch : HEAD

Added sentinel GCC attribute to *_strconcat() functions. Added format printf-attribute to vprintf-type functions. --HG-- branch : HEAD

Added p_free_and_null() and did some cleanups. --HG-- branch : HEAD

Added i_free_and_null() macro and first use case. --HG-- branch : HEAD

Make i_free(), p_free() and pool_unref() calls also set the given parameter to NULL. --HG-- branch : HEAD

Added old_size parameter to p_realloc() - we rarely need it and this way each allocation takes 8 bytes less memory. --HG-- branch : HEAD

Naming style changes, finally got tired of most of the typedefs. Also the previous enum -> macro change reverted so that we don't use the highest bit anymore, that's incompatible with old indexes so they will be rebuilt. --HG-- branch : HEAD

Use unsigned char* when accessing non-NUL terminating strings. Compiler warnings would then notify about accidentally passing them to functions which require them NUL-terminated. Changed a few functions to use void* to avoid unneeded casting. --HG-- branch : HEAD

String function cleanups. Allocating 0 bytes of memory is treated as error now, it could be an integer overflow. Moved printf_string_upper_bound() to it's own file, so strfuncs.c can be with MIT license. --HG-- branch : HEAD

We shouldn't cast sizeof() to unsigned int. --HG-- branch : HEAD

size_t fixes for lib/. Changed OFF_T_FORMAT to PRIuOFF_T which is more C99-like. --HG-- branch : HEAD

Message data is parsed in blocks (no longer entirely mmap()ed). Several IOBuffer changes. All mixed signed/unsigned comparisions were fixed so code can now be compiled with gcc's -W flag. mbox support is broken currently, and there's most likely several other problems too. --HG-- branch : HEAD

Initial revision --HG-- branch : HEAD