Turn INET functions into a function collection plugin

Let's move the following SQL functions:

• INET_ATON()
• INET_NTOA()
• INET6_ATON()
• INET6_NTOA()
• IS_IPV4()
• IS_IPV6()
• IS_IPV4_COMPAT()
• IS_IPV4_MAPPED()
  into a mandatory function collection plugin.

The source files:

• sql_type_inet.h
• sql_type_inet.cc
• item_inetfunc.h
• item_inetfunc.cc
  will move from /sql/ to /plugin/type_inet/.

Later we'll also put the INET6 data type implementation into the same directory.