gpgme-tool: Use membuf functions to build up strings.

gpgme-tool: Use membuf functions to build up strings.

* src/gpgme-tool.c (clear_membuf, init_membuf, put_membuf)
(put_membuf_str, get_membuf, peek_membuf): Add membuf functions.
Take from GnuPG master's common/membuf.[ch] and patch for our use.
(result_xml_escape): Rewrite using new functions.
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First counting, then allocating, and finally copying data is prone to
errors.  We better use the membuf functions which make it much easier.