Before this patch, two macros were used to guard IPv6 features in curl
sources: `ENABLE_IPV6` and `USE_IPV6`. This patch makes the source use
the latter for consistency with other similar switches.
`-DENABLE_IPV6` remains accepted for compatibility as a synonym for
`-DUSE_IPV6`, when passed to the compiler.
`ENABLE_IPV6` also remains the name of the CMake and `Makefile.vc`
options to control this feature.
Closes#13349
e411c98f70