add support for --html

* bootstrap.conf: We need the "execute" module.
* src/files.h, src/files.c (spec_html_file, html_flag): New.
* src/getargs.h, src/getargs.c (--html): New.
* src/print-xml.h, src/print-xml.c (print_html): New.
* src/main.c: Use them.
* tests/output.at, tests/report.at: Check --html.

lib/.gitignore

@@ -67,6 +67,8 @@
 /errno.in.h
 /error.c
 /error.h
+/execute.c
+/execute.h
 /exitfail.c
 /exitfail.h
 /fatal-signal.c