Eliminate duplicate afterinstallconfigs target

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.

MFC after:	2 weeks
Reviewed by:	bapt, brd
Differential Revision:	https://reviews.freebsd.org/D20330

lib/libc/gen/Makefile.inc

@@ -552,7 +552,8 @@ MLINKS+=wordexp.3 wordfree.3
 
 .include <src.tools.mk>
 
-afterinstallconfig:
+afterinstallconfig: install-passwd
+install-passwd: .PHONY
 .if ${MK_TCSH} == "no"
 	sed -i "" -e 's;/bin/csh;/bin/sh;' ${DESTDIR}/etc/master.passwd
 .endif

lib/libc/net/Makefile.inc

@@ -124,8 +124,9 @@ SRCS+=	hesiod.c
 MAN+=	hesiod.3
 .endif
 
+afterinstallconfig: modify-nsswitch-conf
+modify-nsswitch-conf: .PHONY
 .if ${MK_NIS} == "no"
-afterinstallconfig:
 	sed -i "" -e 's/.*_compat:/# &/' -e 's/compat$$/files/' \
 		${DESTDIR}/etc/nsswitch.conf
 .endif