Add option to set protocol-specific parameters

There are a number of protocol-idiosyncratic parameters which sometimes need to be set to specific values to comply with specific VPN's host identification or Trojan mechanisms.

• For AnyConnect, we have mobile_{platform_version,device_type,device_uniqueid,mac_address} (somewhat misleadingly named because these are not mobile-specific). We already have the openconnect_set_mobile_info API function to set these, but no CLI mechanism to invoke it.
• For Juniper/Pulse, we have a few parameters that may need to be set for TNCC to function, including TNCC_DEVICE_ID, TNCC_PLATFORM (e.g. Windows 10.1 SP2 x64 blah blah) and TNCC_HWADDR`.
• For GlobalProtect, we have hostid, osname, and a few other parameters that appear sprinkled throughout the protocol and might be needed by some VPNs for a specific client to be accepted.hostid=BLAHBLAHBLAHBLAH"

There are a few common ones:

• some kind of client device identifier (already have mobile_device_uniqueid) which is "opaque"/"unique" as opposed to the transparent local hostname (localname)
• some kind of detailed platform identifier (already have mobile_platform_version) which is "finer-grained" than the coarse Linux/macOS/Windows (platname)
• MAC address (already have mobile_mac_adress)

Rather than trying to create new CLI options and APIs for new ones as we find them, I propose that we recognize that these are only required by a minority of VPNs, fundamentally idiosyncratic, and just a random grab-bag of keys/values that we need to echo to servers that require them.

We should add a CLI option, something like --local-attribute (to go along with --local-hostname) to set arbitrary key-value pairs which can be used by each protocol as needed.

See https://lists.infradead.org/pipermail/openconnect-devel/2020-April/005667.html for more discussion.