Add support for rfc8572 DHCP option encoding.

[gmacf]

Closes #116

[coveralls]

Pull Request Test Coverage Report for Build 8063303825

Details

• -34 of 34 (0.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 27.787%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
dhcp/main/dhcp.go	0	1	0.0%
dhcp/dhcp.go	0	3	0.0%
dhcp/plugins/bootz/bootz.go	0	30	0.0%

Totals
Change from base Build 7906564035:	-0.3%
Covered Lines:	683
Relevant Lines:	2458

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💛 - Coveralls

[gmacf]

@sam-homa the logic here for encoding the DHCP options is basically taken from our internal DHCP server. I figured you're the most familiar with this.