Re: [tz] tzcode error handling (patch 4 of 4)

Patch 4 adds the ability to print a few extra messages having to do with tzcode's logic for determining how to interpret a requested time zone. There are only two messages so far: "trying %s", when the code is attempting to open a zoneinfo file; and "parsing '%s' as Posix tz string", when it, well, attempts to parse a Posix-style timezone string. That's not much, but it lets you watch, for example, TZ=EST5EDT try both ways but TZ=:EST5EDT only try the file. To enable "verbose debugging", call tz_set_verbose_debug(1), and call tz_set_verbose_debug(0) to turn it off again. Like the rest, this patch is with respect to 2017c. It also depends on Patch 1 (but not 2 or 3). Steve Summit scs@eskimo.com