Local file implementation of secret driver API

Local file implementation of secret driver API

This implementation stores the secrets in an unencrypted text file,
for simplicity in implementation and debugging.

(Symmetric encryption, e.g. using gpgme, will not be difficult to add.
Because the TLS private key used by libvirtd is stored unencrypted,
encrypting the secrets file does not currently provide much additional
security.)

* include/libvirt/virterror.h, src/virterror.c (VIR_ERR_NO_SECRET): New
  error number.
* po/POTFILES.in, src/Makefile.am: Add secret_driver.
* bootstrap: Use gnulib's base64 module.
* src/secret_driver.c, src.secret_driver.h, src/libvirt_private.syms:
  Add local secret driver.
* qemud/qemud.c (qemudInitialize): Use the local secret driver.