Eliminate duplicate `afterinstallconfigs` target

[ngie]

Summary

Define separate dependent targets which afterinstallconfigs relies on, in
order to modify ${DESTDIR}/etc/master.passwd and
${DESTDIR}/etc/nsswitch.conf.

Mark these targets .PHONY, since they manipulate configurations on the fly and
the generation logic isn't 100% defined in terms of the source files/logic,
and is variable, based on MK_foo flags.

Test Plan

Before:

$ (cd lib/libc; make buildenv)
[Creating objdir /usr/obj/home/ngie/svn/freebsd/base/projects/runtime-coverage-v2/amd64.amd64/lib/libc...]
make: "/home/ngie/svn/freebsd/base/projects/runtime-coverage-v2/lib/libc/net/Makefile.inc" line 130: warning: duplicate script for target "afterinstallconfig" ignored
make: "/home/ngie/svn/freebsd/base/projects/runtime-coverage-v2/lib/libc/gen/Makefile.inc" line 552: warning: using previous script for "afterinstallconfig" defined here
$

After:

$ (cd lib/libc; make buildenv)
Entering world for amd64:amd64
$

[ngie]

Still an issue:

$ (cd ~/svn/freebsd/base/head/; svnversion; make -C lib/libc buildenv)
352519
make: "/home/ngie/svn/freebsd/base/head/lib/libc/net/Makefile.inc" line 130: warning: duplicate script for target "afterinstallconfig" ignored
make: "/home/ngie/svn/freebsd/base/head/lib/libc/gen/Makefile.inc" line 554: warning: using previous script for "afterinstallconfig" defined here
Entering world for amd64:amd64

[brd]

Thanks, sorry I missed the notification about this review before.