Get rid unneccessary useless calls to lseek(2) from getgrent(3).

Get rid unneccessary useless calls to lseek(2) from getgrent(3).
Looks like this:

 fstatat(AT_FDCWD,"/etc/nsswitch.conf",{ mode=-rw-r--r-- ,inode=2167001,size=390,blksize=32768 },0x0) = 0 (0x0)
 open("/etc/group",O_RDONLY|O_CLOEXEC,0666)      = 3 (0x3)
 fstat(3,{ mode=-rw-r--r-- ,inode=2166927,size=919,blksize=32768 }) = 0 (0x0)
-lseek(3,0x0,SEEK_CUR)                           = 0 (0x0)
-lseek(3,0x0,SEEK_SET)                           = 0 (0x0)
 read(3,"# $FreeBSD: release/10.0.0/etc/g"...,32768) = 919 (0x397)
 close(3)                                        = 0 (0x0)

Reviewed by:	kib
Approved by:	re (gjb)
MFC after:	2 weeks
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D17571