MFC r320737, r320914

MFC r320737, r320914

r320737:
Fix cleanup in lib/libc/gen/setdomainname_test

ATF cleanup routines run in separate processes from the tests themselves, so
they can't share global variables.

Also, setdomainname_test needs to be is_exclusive because the test cases
access a global resource.

PR:		219967
Reviewed by:	ngie
Sponsored by:	Spectra Logic Corp
Differential Revision:	https://reviews.freebsd.org/D11188

r320914:
Remove an extraneous strlen from t_setdomainname.c

Reported by:	Coverity
CID:		1377568
X-MFC-With:	320737
Sponsored by:	Spectra Logic Corp