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.