ShellPkg: Fixes for timezone handling and 'date -sfo'

- Update 'date -sfo' format to match UEFI Shell 2.1 spec
- Fixes to correctly initialize Second
- Set correct sign when setting timezone with the 'time -tz' command.  Now matches UEFI spec calculation of "Localtime = UTC - TimeZone"
- Display "LOCAL" when TimeZone == EFI_UNSPECIFIED_TIMEZONE
- Allow a timezone of '_local' to be provided by user
- Better invalid command line checking

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chris Phillips <chrisp@hp.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15840 6f19259b-4bc3-4df7-8a09-765794883524