[tz] Support for Etc/GMT -style zones ids with partial hours

[Paul Eggert]

On 10/21/20 11:45 AM, Neil Fuller wrote:
> To be honest, just adding new "Etc/GMT-5:30" identifiers could also cause
> problems (not just for Android), but it seems likely it could be made to
> work without too much effort on the Android side. Even if there was
> enthusiasm here, I would want to try it out and would need to consider the
> effect on older releases of Android

One way to do that would be to add a file etc-rare that contains these 
less-commonly-used and/or problematic offsets. Something like the attached, say. 
You can try this out in your Android builds. Not sure whether it'd be worthwhile 
putting it into tzdb, as this sort of thing could be a neverending story and 
anyway it really should be done via a POSIX-style TZ support.
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# tzdb data for less commonly used fixed-offset zones

# This file is in the public domain, so clarified as of
# 2009-05-17 by Arthur David Olson.

# This file is like the etcetera file, except that it supports
# less commonly used offsets.

# These non-integer offsets are used in civil times somewhere as of
# the year 2020.
Zone	Etc/GMT-13:45	13:45	-	+1345
Zone	Etc/GMT-12:45	12:45	-	+1245
Zone	Etc/GMT-10:30	10:30	-	+1030
Zone	Etc/GMT-9:30	9:30	-	+0930
Zone	Etc/GMT-8:45	8:45	-	+0845
Zone	Etc/GMT-6:30	6:30	-	+0630
Zone	Etc/GMT-5:45	5:45	-	+0545
Zone	Etc/GMT-5:30	5:30	-	+0530
Zone	Etc/GMT-4:30	4:30	-	+0430
Zone	Etc/GMT-3:30	3:30	-	+0330
Zone	Etc/GMT+2:30	-2:30	-	-0230
Zone	Etc/GMT+3:30	-3:30	-	-0330

# These integer offsets might be used by astronomers.
Zone	Etc/GMT-23	23	-	+23
Zone	Etc/GMT-22	22	-	+22
Zone	Etc/GMT-21	21	-	+21
Zone	Etc/GMT-20	20	-	+20
Zone	Etc/GMT-19	19	-	+19
Zone	Etc/GMT-18	18	-	+18
Zone	Etc/GMT-17	17	-	+17
Zone	Etc/GMT-16	16	-	+16
Zone	Etc/GMT-15	15	-	+15
# Etc/GMT-14 through Etc/GMT+12 are in etcetera.
Zone	Etc/GMT+13	-13	-	-13
Zone	Etc/GMT+14	-14	-	-14
Zone	Etc/GMT+15	-15	-	-15
Zone	Etc/GMT+16	-16	-	-16
Zone	Etc/GMT+17	-17	-	-17
Zone	Etc/GMT+18	-18	-	-18
Zone	Etc/GMT+19	-19	-	-19
Zone	Etc/GMT+20	-20	-	-20
Zone	Etc/GMT+21	-21	-	-21
Zone	Etc/GMT+22	-22	-	-22
Zone	Etc/GMT+23	-23	-	-23