Fixes in offload-glx / offload-vulkan

[blechta]

This PR fixes two issues:

1. Fix handling of white space in arguments for offload-glx / offload-vulkan

before the patch:

$ offload-glx bash -c 'echo this should be printed'
offload-glx: launched.
  - nvidia-settings and prime-select detected.
  - prime-supported: Yes
offload-glx: NVIDIA Optimus supported.
Checking: on-demand
  - on-demand loaded: n/a
  - prime-supported: Yes
  - nvidia-detector: nvidia-driver-440
  - prime-select: on-demand detected.
  - xrandr: NVIDIA GPU screen detected.
  - on-demand: is supported
offload-glx: NVIDIA On-Demand is enabled.
offload-glx: Launching "bash -c echo this should be printed" with NVIDIA offload.

after the patch:

$ offload-glx bash -c 'echo this should be printed'
offload-glx: launched.
  - nvidia-settings and prime-select detected.
  - prime-supported: Yes
offload-glx: NVIDIA Optimus supported.
Checking: on-demand
  - on-demand loaded: n/a
  - prime-supported: Yes
  - nvidia-detector: nvidia-driver-440
  - prime-select: on-demand detected. tt 
  - on-demand: is supported
offload-glx: NVIDIA On-Demand is enabled.
offload-glx: Launching ['bash', '-c', 'echo this should be printed'] with NVIDIA offload.
this should be printed

2. Use exec instead of popen in offload-glx / offload-vulkan. This ensures that the wrapper is terminated and replaced by the target process, thus preventing any undesired interaction.

MWE:

offload-glx bash -c 'trap "echo got ctrl+c but will not quit" SIGINT; while true; do :; done'

This program should keep running after hitting CTRL+C, which is ensured by this patch.

[blechta]

@flexiondotorg, could you have a look? 😃

[flexiondotorg]

Thank you for contributing!