Current code in sysctl proc.vmmap, which intent is to calculate the
amount of resident pages, in fact calculates the amount of installed
pte entries in the region.  Resident pages which were not soft-faulted
yet are not counted.

Calculate the amount of resident pages by looking in the objects chain
backing the region.

Add a knob to disable the residency calculation at all.  For large
sparce regions, either previous or updated algorithm runs for too long
time, while several introspection tools do not need the (advisory) RSS
value at all.

PR:	kern/188911
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Implement sysconf(_SC_GETGR_R_SIZE_MAX) and sysconf(_SC_GETPW_R_SIZE_MAX).

Reported by:	Dmitry Sivachenko <trtrmitya@gmail.com>
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week