gpg-interface: use left shift to define GPG_VERIFY_*

gpg-interface: use left shift to define GPG_VERIFY_*

In "gpg-interface.h", the definitions of the GPG_VERIFY_* boolean flags
are currently using 1, 2 and 4 while we often prefer the bitwise left
shift operator, `<<`, for that purpose to make it clearer that they are
boolean.

Let's use the left shift operator here too. Let's also fix an indent
issue with "4" while at it.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>