ignore lib/base/base64.c in Git

lib/base/base64.c is an auto-generated file. Add it to the list in
.gitignore
roken: don't ignore HAVE_OPENPTY on linux

openpty() is not available on all Linux distributions. Trust autoconf's
determination for HAVE_OPENPTY instead of unconditionally using
openpty() on all Linux.
Properly terminate ifdef conditional in krb5-types.h
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