Revert "vfork() isn't functional on DOS"

This reverts commit 91173aba25266194408c9d401e511395a732c2b5.

As @icculus mentioned: I'd rather we not get into the business of making fixes by tweaking values in the Preseed files. Were there no preseed file, CMake would correctly record that the vfork function exists, so if we're really hitting the vfork() codepath on DOS, we should stop that in either CMakeLists.txt itself, or in src/process.

cmake/PreseedDOSCache.cmake

@@ -141,7 +141,7 @@ if(CMAKE_SYSTEM_NAME STREQUAL "DOS")
       set(LIBC_HAS_TRUNC                                 "1"   CACHE INTERNAL "Have symbol trunc")
       set(LIBC_HAS_TRUNCF                                "1"   CACHE INTERNAL "Have symbol truncf")
       set(LIBC_HAS_UNSETENV                              "1"   CACHE INTERNAL "Have symbol unsetenv")
-      set(LIBC_HAS_VFORK                                 ""    CACHE INTERNAL "Have symbol vfork")
+      set(LIBC_HAS_VFORK                                 "1"   CACHE INTERNAL "Have symbol vfork")
       set(LIBC_HAS_VSNPRINTF                             "1"   CACHE INTERNAL "Have symbol vsnprintf")
       set(LIBC_HAS_VSSCANF                               "1"   CACHE INTERNAL "Have symbol vsscanf")
       set(LIBC_HAS_WCHAR_H                               "1"   CACHE INTERNAL "Have include wchar.h")