sysutils/apcupsd: Several improvements

Add support for running multiple instances of apcupsd[1]
Add an option to use shutdown -p rather than shutdown -h[2]
Set as @sample several scripts that can be customized[3]
Add an option for build with MODBUS USB driver support (Enabled by default)[4]
Set LICENSE_FILE [5]
Sort OPTIONS and make a small adjustment in pkg-message
Take maintainer'ship

PR: 180336 [1]
PR: 187924 [2][3]
PR: 215809 [4]
PR: 220002 [5]

Submitted by: leres@ee.lbl.gov [1]
Submitted by: tdb [3]
Submitted by: ipluta@wp.pl [4]
Submitted by: takefu@airport.fm [5]

Approved by: garga (mentor)
Differential Revision: https://reviews.freebsd.org/D11224