mktag.c: tweak validation of tagger field and adjust test script

mktag.c: tweak validation of tagger field and adjust test script

Update the verify_tag() function to remove an unnecessary test, and add
additional check for angle brackets in the name and email field, and
spaces in the email field. The timestamp and timezone sections are made
more straight forward by using strspn().

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>