Fix a bug in tab stop handling in SourceWidget.

 * Currently, every tab character is as wide as kTabStopSize spaces.

 * The division by m_tabStopPx was supposed to be truncated to an integer.
   Originally, it *was* truncated, but this was broken by the int-to-qreal
   coordinate change on 2013-01-17 (9f7c9ca8a0).
FreeBSD btrace: Read argv using kvm instead of sysctl.

 * I do not know why, but sysctl returned an empty argv in some cases.
   Specifically, while building openssl-1.0.1c on PC-BSD 9.0, sysctl
   returned an empty argv for some invocations of "sh".

 * FreeBSD also has a "kvm" interface, which FreeBSD's ps utility uses
   to query a process' command-line.  It seems to work in cases where
   sysctl did not, so use it instead.