Add key+=append syntax to sysrc(8).

Reviewed by:	shurd
MFC after:	3 days
X-MFC-to:	stable/10 stable/9
Finish off lib/libc/stdlib/t_strtod.c port by checking for "y" twice on
FreeBSD, and always assume long long double exists on FreeBSD

Submitted by: pho
Follow-up to r255036; remove beforeinstall directives from bsdconfig(8)
Makefile's, fixing concurrent installworld (`make -j17 installworld').

Thanks to:	delphij, emaste
Reviewed by:	delphij
MFC after:	3 days
X-MFC-to:	stable/10, stable/9
Add Makefile snippet to ease porting NetBSD testcases to FreeBSD from
contrib/netbsd-tests

This Makefile snippet handles polluting testcases with -lnetbsd, specific
headers for ATF version differences, and does necessary rewriting for the
testcases to match the format discussed on the TestSuite wiki page
(t_<foo> -> <foo>_test)

One must define SRCTOP (inspired by projects/bmake), OBJTOP, and TESTSRC
(e.g. contrib/netbsd-tests/lib/libc/gen) to use the Makefile snippet

Test programs are specific either via NETBSD_ATF_TESTS_C or NETBSD_ATF_TESTS_SH

C++ analogs aren't currently implemented.

The imported testcases will be cleaned up to use this Makefile snippet pseudo
"API".