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