Bug#29590300 ABI INCOMPATIBILITIES DUE TO WL#12571:SUPPORT FULLY QUALIFIED HOSTNAMES LONGER THAN 60 CHARACTERS

The macro HOSTNAME_LENGTH should not be part of ABI check. So this patch
moves the macro out from mysql_com.h to avoid ABI check.

- HOSTNAME_LENGTH is moved from mysql_com.h to include/my_hostname.h.

- USER_HOST_BUFF_SIZE is moved from mysql_com.h to sql/auth/auth_common.h.

- PORTNUMBER_LENGTH is moved from mysql_com.h to include/my_hostname.h.
  This was added by wl12571 resently.

Change-Id: I265f965dcd9093a87a1fdaca616693c02f4f9963