pkg/53055: pkg_add should sync after adding a package

[coypu%sdf.org@localhost]

>Number:         53055
>Category:       pkg
>Synopsis:       pkg_add should sync after adding a package
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    pkg-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Feb 25 16:30:00 +0000 2018
>Originator:     coypu
>Release:        NetBSD 8.0_BETA
>Organization:
>Environment:
NetBSD planets 8.0_BETA NetBSD 8.0_BETA (GENERIC) #0: Sun Feb 25 13:32:49 IST 2018  fly@planets:/home/fly/obj8/sys/arch/amd64/compile/GENERIC amd64

>Description:
While testing PR 53053, pkg corruption was really bad if the machine crashes right after adding a package.
calling sync after manually, it was fine.

sync is pretty cheap, we should do it for normal pkg addition.
>How-To-Repeat:

>Fix: