Refactor and Better Document protocolIntegers() and protocolStrings() Usage

[bflad]

Description

The protocolIntegers() function is used to return a map[string]int of IANA protocol keywords to protocol numbers, while the protocolStrings() function is used to return the opposite (map[int]string) of the same information.

The usage of these functions is a little awkward and sometimes saved into a nondescript p variable, e.g. p, ok = protocolIntegers()[protocol]

We should consider:

• Improving the naming and usability of this information
• Improving the output variable naming if they are to remain variables/functions

References

• resource/aws_network_acl_rule: Properly handle ICMP code and type with IPv6 ICMP (protocol 58) #6263 (comment)
• resource/aws_network_acl: Properly handle ICMP code and type with IPv6 ICMP (protocol 58) #6264 (comment)
• https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml
• https://github.com/terraform-providers/terraform-provider-aws/blob/1595196ec55d96c44417012a580912a50e63acab/aws/network_acl_entry.go#L89-L247
• https://github.com/terraform-providers/terraform-provider-aws/blob/1595196ec55d96c44417012a580912a50e63acab/aws/network_acl_entry.go#L46

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[ghost]

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