Replace .cvsignore files with a project-wide .gitignore file.

* .cvsignore: Replace with .gitignore.
* include/private/.cvsignore: Remove.

.gitignore

@@ -0,0 +1,60 @@
+# Ignored files in bdwgc Git repo.
+
+# Binary files:
+/*.dll
+/*.exe
+/*.la
+/*.lib
+/*.lo
+/*.o
+/*.obj
+
+/.deps/
+/.libs/
+/Makefile
+/add_gc_prefix
+/bdw-gc.pc
+/config.log
+/config.status
+/cordtest
+/core
+/de
+/gc-*
+/gcname
+/gctest
+/hugetest
+/if_mach
+/if_not_there
+/initsecondarythread
+/leaktest
+/libtool
+/middletest
+/realloc_test
+/setjmp_test
+/smashtest
+/staticrootstest
+/test_cpp
+/threadkey_test
+/threadleaktest
+/threadlibs
+
+/include/private/config.h
+/include/private/config.h.in~
+/include/private/stamp-h1
+
+# Rarely generated files (mostly by some Win/DOS compilers):
+/*.bsc
+/*.csm
+/*.err
+/*.exp
+/*.lb1
+/*.lnk
+/*.map
+/*.out
+/*.pdb
+/*.rbj
+/*.res
+/*.sbr
+/*.stackdump
+/*.sym
+/*.tmp