Fix up "Postgres-style" time interval representation when fields have

Thomas G. Lockhart <lockhart@fourpalms.org>

Commit: 6acdebbd3a6e516e26b9d5ff7c51e51802e7345b
Author: Thomas G. Lockhart <lockhart@fourpalms.org>
Date: 2001-01-18T07:22:43Z
Releases: 7.1.1
Fix up "Postgres-style" time interval representation when fields have
 mixed-signs. Previous effort left way too many minus signs, and was at
 least as broken as the one before that :(
Clean up "ISO-style" time interval representation to omit zero fields if
 there is at least one non-zero field. Supress some leading plus signs
 when not necessary for clarity.
Replace every #ifdef __CYGWIN__ block with a cleaner TIMEZONE_GLOBAL macro
 defined in datetime.h.

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