Prevent @Label completion unless a user has disabled hostcomplete

The hostcomplete shopt will complete any "word@" with a hostname, which is a problem for us,
since Perforce uses @ to specify a file at a given label, e.g. "file@label".
bash-completion disables the hostcomplete shopt by default and offers an alternative for hostname
completion for programs like ssh and scp.  But if you don't have bash-completion installed,
hostcomplete could be enabled, and this will do weird things if we try to offer completion with
the @ symbol.

This change prevents us from suggesting the @ symbol as a completion option if hostcomplete is
enabled, and prevents any attempts to complete a word with the @ symbol.

Disabling hostcomplete with `shopt -u hostcomplete` will bring back the @Label completion functionality.

Include bash-completion functionality

If bash-completion is not installed, use the functionality from bash-completion 1.3

Rename p4_completion to p4-completion

p4-completion with a dash matches the bash-completion

Update Mac OS name