Add sysrc(8) support for "rc.conf.d" file(s) when given "-s name" to

Add sysrc(8) support for "rc.conf.d" file(s) when given "-s name" to
indicate service(8) script. While here, add "-l" option for listing the
set of configuration file(s) considered (in order; separated by space).
Also add "-L" for exploring all configuration files and "-E" to omit
files that don't exist from operations.

Differential Revision:	https://reviews.freebsd.org/D3551
Reviewed by:	allanjude
MFC after:	1 week
X-MFC-to:	stable/10
Relnotes:	yes