build: prefer `USE_IPV6` macro internally (was: `ENABLE_IPV6`)

build: prefer `USE_IPV6` macro internally (was: `ENABLE_IPV6`)

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