BSD: Only use BSD as variable for target BSD system

Instead of having multiple boolean variables, we only have one
string variable: BSD with the name of the target BSD system.

Relates: #23853

Help/manual/cmake-variables.7.rst

@@ -324,8 +324,6 @@ Variables that Describe the System
    /variable/CMAKE_SYSTEM_PROCESSOR
    /variable/CMAKE_SYSTEM_VERSION
    /variable/CYGWIN
-   /variable/DRAGONFLYBSD
-   /variable/FREEBSD
    /variable/GHSMULTI
    /variable/IOS
    /variable/LINUX
@@ -344,8 +342,6 @@ Variables that Describe the System
    /variable/MSVC_TOOLSET_VERSION
    /variable/MSVC_VERSION
    /variable/MSYS
-   /variable/NETBSD
-   /variable/OPENBSD
    /variable/UNIX
    /variable/WIN32
    /variable/WINCE