Fix some static analysis warnings

With HAVE_PTHREAD_SETNAME_NP

In Mu::parse_date_time, when provided with an empty string, return
time_t_max instead of G_MAXINT64. For systems with a 64-bit time_t, there
is no difference. With a 32-bit time_t it caused a test to fail:

    not ok /utils/date-basic - ERROR:../mu-1.12.4/lib/utils/tests/test-utils.cc:92
    void test_date_basic(): assertion failed
    (parse_date_time(std::get<0>(test), std::get<1>(test)).value_or(-1)
      == std::get<2>(test)): (18446744073709551615 == 2147483647)

This edge case probably only affected the test, as when other parts of
the application call parse_date_time (e.g. mu-server.cc and
mu-query-processor.cc), they check if the input string is empty first.

Esp. for systems with 32-bit time_t.

Determine if a string has wordbreaks in a mostly Xapian-compatible way.
We need this to determine what strings should be considered "phrases".

Do not removing combining characters from scripts without explicit word
boundaries, such as those for CJK.

Reuse some Xapian code for that.

Parsing dates known to be in UTC.

Use the new fmt-based formatting.

It's faster; make "mu find" ~5-10% faster, and removes some code we no
longer need.