lib: fix AIX build issues

lib: fix AIX build issues

- memdebug: replace keyword `malloc` with `__malloc__` to
  not interfere with envs where `malloc` is redefined. Also apply
  the fix to `alloc_size`.
  Fixes:
  ```
  lib/memdebug.h:107:13: warning: unknown attribute 'vec_malloc' ignored [-Wunknown-attributes]
  CURL_EXTERN ALLOC_FUNC FILE *curl_dbg_fdopen(int filedes, const char *mode,
              ^~~~~~~~~~
  lib/memdebug.h:37:37: note: expanded from macro 'ALLOC_FUNC'
  # define ALLOC_FUNC __attribute__((malloc))
                                     ^~~~~~
  /usr/include/stdlib.h:753:16: note: expanded from macro 'malloc'
  #define malloc vec_malloc
                 ^~~~~~~~~~
  ```

- memdebug: always undef before defining.
  Also do this for the rest of functions redefined in the same block.
  Avoids warning on AIX:
  ```
  lib/memdebug.h:117:9: warning: 'malloc' macro redefined [-Wmacro-redefined]
  #define malloc(size) curl_dbg_malloc(size, __LINE__, __FILE__)
          ^
  /usr/include/stdlib.h:753:9: note: previous definition is here
  #define malloc vec_malloc
          ^
  ```

- easy: fix `-Wformat` warning on AIX by adding a cast.
  ```
  lib/easy.c:608:47: warning: format specifies type 'int' but the argument has type 'long' [-Wformat]
  "%" CURL_FORMAT_SOCKET_T ")", fds[i].fd);
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
  ```

- if2ip: silence compiler warning inside AIX system header.

  ```
  /lib/if2ip.c:219:19: warning: signed shift result (0x80000000) sets the sign bit of the shift expression's type ('int') and becomes negative [-Wshift-sign-overflow]
  if(ioctl(dummy, SIOCGIFADDR, &req) < 0) {
                  ^~~~~~~~~~~
  /usr/include/sys/ioctl.h:401:26: note: expanded from macro 'SIOCGIFADDR'
  #define SIOCGIFADDR (int)_IOWR('i',33, struct oifreq) /* get ifnet address */
                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /usr/include/sys/ioctl.h:174:23: note: expanded from macro '_IOWR'
  #define _IOWR(x,y,t) (IOC_INOUT|((sizeof(t)&IOCPARM_MASK)<<16)|(x<<8)|y)
                        ^~~~~~~~~
  /usr/include/sys/ioctl.h:168:20: note: expanded from macro 'IOC_INOUT'
  #define IOC_INOUT (IOC_IN|IOC_OUT)
                     ^~~~~~
  /usr/include/sys/ioctl.h:167:28: note: expanded from macro 'IOC_IN'
  #define IOC_IN (0x40000000<<1) /* copy in parameters */
                  ~~~~~~~~~~^ ~
  ```

Ref: https://curl.se/dev/log.cgi?id=20240808180420-3809007
Assisted-by: Dan Fandrich
Closes #14464