* test-suite/tests/signals.test: Update setitimer tests to sloppily
match both times -- it seems that the interval can also be sloppy on
some platforms, sadly.
* doc/ref/posix.texi (Signals): Fix the documentation for setitimer; it
was wrong.
* libguile/scmsigs.c (pack_tv): New helper. Allow usecs >= 1e6.
(unpack_tv): New helper.
(scm_setitimer): Use the new helpers.
* test-suite/tests/signals.test: Add setitimer tests.