Fix incremental builds involving non-root users with read-only source files.

Makefiles should not assume that source files can be overwritten.  This is the
common case for Perforce source trees.

This is a followup commit to r211243 in the same vein.

MFC after:	1 month
Sponsored by:	Spectra Logic
MFSpectraBSD:	r1036319 on 2014/01/29, r1046711 on 2014/03/06
Accidentially, ng_ppp changes leaked out with r271770. The proper
commit message should have been:

In case if ng_ppp(4) runs only IPv4, with one link, w/o any
encryption and compression, set faster rcvhook method.

Submitted by:	Dmitry Luhtionov <dmitryluhtionov gmail.com>
While not too late rename 'ifnet_counter' to 'ift_counter'. One of the
imporant moments that we discussed with Marcel and Anuranjan was that
a converted driver should return false for 'grep ifnet if_driver.c' :)

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Fix incremental builds involving non-root users with read-only source files.
This is a followup commit to r271771.

MFC after:	1 month
Fix incremental builds involving non-root users with read-only source files.
This is a followup commit to r271771.

MFC after:	1 month