Prevent zombie processes (#8774)

Using Systemcall::startscript() with Starttype::DontWait used to create
zombie processes, as nobody would collect them. This patch starts those
processes as detached, hence preventing them from becoming zombies.

In addition to #8774, this bug was also reported here:
https://bugs.launchpad.net/ubuntu/+source/lyx/+bug/1096666

Patch from Guy Rutenberg. For reference, see this email thread:
http://www.mail-archive.com/lyx-devel@lists.lyx.org/msg180034.html
Merge branch 'master' of git://git.lyx.org/lyx

# By Scott Kostyshak
# Via Scott Kostyshak
* 'master' of git://git.lyx.org/lyx:
  Prevent zombie processes (#8774)