croutonclip: Fix signal handling for bash 4.3

Behaviour of read was changed in bash 4.3, and read does not get
interrupted by signals anymore (affects mostly arch, as trusty
uses dash by default).

Switch to using wait, since that gets interrupted, and we have
children in any case. Incidentally, this simplifies the code
quite a bit.
croutonclip: Refactor triggering code

For some reason bash 4.3.11 (trusty) segfaults often with the
previous version of the code, presumably because traps/sigaction
are not reset properly in children.

We refactor the code to receive signals in a subshell, and print
a line everytime a signal is received. We then read these lines
from the other end of a pipe, and transfer the clipboard.

Tested:
 - arch bash 4.3.24
 - trusty bash 4.3.11 / dash
 - precise bash 4.2.25 / dash (Xephyr)
Merge pull request #1025 from drinkcat/croutonclip-fix-signal

Croutonclip: fix signal handling