Because of a regression introduced by bug#19027 the option --enable-foobar
doesn't work anymore for any plugin 'foobar'. The reason is that plugin
names are tristate options variables with optional parameters and integer
values are not accepted. Since the 'enable' prefix attempts to assign '1'
to the option the operation fails.

This patch translates any number n assigned to a plugin variable of type ENUM
to be the corresponding enumerated item. As a side effect --enable-foobar and
--disable-foobar will also start working again.

mysys/my_getopt.c:
  * setval now accepts integer values for option variables of type ENUM.