Flags is an overloaded term in this context. It generally is used to
refer to command line flags. This struct, however, is a data object
used as input to the construction.
It happens to be partially populated by command line flags, but
otherwise has very little to do with them.
Renaming this struct should make the actual responsibility of this struct
more obvious, and remove the possibility that it is confused with
command line flags.
This change is in preparation for adding additional fields to
BuilderOpts.
I spent some time today on my local Mac to figure out why Consul 1.6.3+
was not accepting limits.http_max_conns_per_client.
This adds an explicit check on number of file descriptors to be sure
it might work (this is no guarantee as if many clients are reaching
the agent, it might consume even more file descriptors)
Anyway, many users are fighting with RLIMIT_NOFILE, having a clear
message would allow them to figure out what to fix.
Example of message (reload or start):
```
2020-03-11T16:38:37.062+0100 [ERROR] agent: Error starting agent: error="system allows a max of 512 file descriptors, but limits.http_max_conns_per_client: 8192 needs at least 8212"
```
Daniel Nephin