[tmux.conf] Should not auto attach to first session at startup

[rucker]

Commit c625124 introduced logic whereby tmux will automatically attach to the first existing tmux session if there is one. This behavior is acceptable in situations like starting up a terminal session after all had previously been closed; However it is undesirable when for example opening a terminal inside an IDE, or opening a new tab in the existing terminal emulator.

Two options for remedying this are:

1. Revert the behavior change from c625124
2. Offer the user a choice of existing session(s) to optionally attach (prompt for input). Note that there may be existing tmux functionality to assist with this option.