Don't call stat(2) on nsswitch.conf(5) every time nsdispatch(3)
and dependent functions (eg getpwname(3)) get called. This can
improve performance of binaries that perform a lot of name
lookups, such as gssd(8). It also matches documented behaviour
of Linux and Solaris.

The old code is left in place, should anyone need it, guarded
by #ifdef NS_REREAD_CONF.

Reviewed by: imp, bcr
MFC after: 2 weeks
Relnotes: yes
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17934