libpromises/bootstrap.c: generate from a .cf file

Mechanism to generate `libpromises/bootstrap.c` from `../masterfies/bootstrap/failsafe.cf`

Simply do `cd libpromises; make failsafe`.  You need Perl installed.

Only one promise is dubious between the old failsafe.cf (the `-` lines) and the new failsafe.cf (the `+` lines).

```
-      "$(sys.workdir)\bin-twin\."
+      "$(sys.workdir)\\bin-twin\\."
       handle => "cfe_internal_bootstrap_update_files_sys_workdir_bin_twin_windows",
       comment => "Make sure we maintain a clone of the binaries and libraries for updating",
-      copy_from => u_cp("$(sys.workdir)\bin\."),
+      copy_from => u_cp("$(sys.workdir)\\bin\\."),
```