Use capitalized names in option description
Use correct command to get installed ports

This fixes a copy-paste error when we completed any port name instead of
installed ones
Explicitly typeset arrays
Better port upgrade completion

When upgrading, pseudo-portnames can be used. Also, add upgrade options to
completion choices.
Add description to each port target

Also, remove targets used only in interactive mode, like 'exit' or 'cd'
No need to set cache policy in the main function

The caching policy style for _port can be set in _port_dispatch, as it is the
one that uses caching.
Use the -q flag when calling port

The -q flag suppresses any output other than port names, eliminating the need to
further modify output.
A better cache policy for _port

This compares modification times of the main PortIndex for all ports and
registry.db for installed ports.
No need to use the ->string format

If we have only one function to call fro completion, just call it. Also, remove
local declaration, since we don't use the ->string form.
_port: complete more subcommands

Add completions for outdated, sync and selfupdate subcommands.
_port: Fix completion of next arguments
Merge pull request #240 from g5pw/master

_port completion enhancement