Make tup monitor use the correct executable

[jonatanolofsson]

In tup monitor, tup uses "tup" as path to tup in the rebuild instead of calling the currently executed version. This commit fetches the current executable (using /proc/self/exe) and uses the same path for the new update process.

[jonatanolofsson]

Note that the executable is not sent to the monitor in argv[0], so this fix is linux(/unix?) specific. Note however that the entire monitor function is linux only, so I figure that's alright.