Fix waitForProcess errors when configuration is recompiled

Previously, xmobar would ignore SIGCHLD, but only when the configuration
is recompiled. This means child processes would not leave zombies, so
that waiting for them to exit (either directly by calling
waitForProcess, or indirectly through another function from
System.Process, like system or readProcessWithExitCode) would produce an
error. This breaks the Alsa, Com (#657) and NotmuchMail plugins, as well
as the <action> tag (#687) and low battery action (#688).

As far as I can tell, bracketing the recompilation in
uninstallSignalHandlers and installSignalHandlers was inherited from
xmonad. In xmonad, SIGPIPE and SIGCHLD are always ignored
(installSignalHandlers is called at the start of xmonad and launch), so
the bracket is necessary to be able to wait for the compiler or build
script to exit. Since xmobar does not otherwise ignore the signals, it
is not necessary to change signal handlers at all during recompilation.
Removing it leaves the default action for SIGCHLD, which fixes the
issues described above.

Fixes #657, #687 and #688.

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