serverutil/upspinserver: don't panic on startup if $HOME not found
serverutil/upspinserver: don't panic on startup if $HOME not found

In environments such as Docker containers, where a process may not be
able to find its home directory, we don't want upspinserver to panic.
It should instead just use a simple default.

Change-Id: I5cbf5a196a814ea9233ef830d91dca0dbecdbf9c
Reviewed-on: https://upspin-review.googlesource.com/11563
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin