Stop using timeout(1)

steam-runtime-tools currently runs several diagnostic tools under timeout(1), which is a bit of a mess for a couple of reasons:

• Canonical's Steam Snap packaging runs under a restrictive AppArmor profile that doesn't allow execve() of /usr/bin/timeout
• Starting a new process is relatively expensive, particularly in an emulation environment like FEX-Emu

Ideally we could do this more like:

• create a new GMainContext
• set it as the thread-default main context
• attach I/O and child process sources to that main context
• also attach a timeout source to that main context
• iterate the main context until the process has exited and the stdout/stderr fds have reached EOF
• pop thread-default main context
• sort out results
• return to caller

@smcv has most of an implementation of this.

This would be easier with GSubprocess and GTask, but those need GLib 2.40.