tzselect: Fix for gawk treating '\.' as plain '.'

When using gawk and Posix TZ in tzselect, this warning is shown:
	awk: cmd. line:1: warning: escape sequence `\.' treated as plain `.'
gawk treats '\.' as '.'
mawk treats '\.' as '\.'
both treat '\\.' as '\\.'
The 2nd and 3rd are corrrect for 'Mm.w.d'.

Test:
gawk 'BEGIN{ date = "\."; print date }'
gawk 'BEGIN{ date = "\\."; print date }'
versus:
mawk 'BEGIN{ date = "\."; print date }'
mawk 'BEGIN{ date = "\\."; print date }'
* tzselect.ksh (date): Fix bug that broke POSIX TZ checking with gawk.
* NEWS: Document this.