ReceivePack: Clarify the check reachable option

This option was mis-named from day 1.  Its not checking that the
objects provided by the client are reachable, its actually doing
a scan to prove that objects referenced by the client are already
reachable through another reference on the server, or were sent
as part of the pack from the client.

Rename it checkReferencedObjectsAreReachable, since we really are
trying to validate that objects referenced by the client's actions
are reachable to the client.

We also need to ensure we run checkConnectivity() anytime this is
enabled, even if the caller didn't turn on fsck for object formats.
Otherwise the check would be completely bypassed.

Change-Id: Ic352ddb0ca8464d407c6da5c83573093e018af19
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>