gpgme-tool: escape special characters in output XML data (<, >, and &).

PGP Signed Part:Undecided
src/gpgme-tool.c (result_xml_escape_replacement, result_xml_escape):
New.
(result_xml_tag_data): Use result_xml_escape() to escape data.

(result_add_error): Use unescaped < and >.

This is a general solution for generating valid XML, but the specific
output that inspired the change was from the KEYLIST command:

<uid>William Trevor King <wking@tremily.us></uid>

Now the uids are properly escaped:

<uid>William Trevor King &lt;wking@tremily.us&gt;</uid>

• Signed-off-by: W. Trevor King <wking@tremily.us>