portability: use `uname -n` instead of `hostname`

startx calls hostname to obtain the hostname, but this is not defined by POSIX, so not guaranteed to work.
(the script already does some checks to deal with different hostname implementations on Linux...)

Using uname -n instead fixes this in a portable way. Patch: startx-uname.diff

See also https://unix.stackexchange.com/questions/199698/uname-n-vs-hostname