mptcpize: use systemd overrides instead of editing vendor provided .service

[candrews]

Is your feature request related to a problem? Please describe.

Currently, mptcpize will modify the vendor provided systemd service file located at /lib/systemd/system/*.service.

This approach is problematic because that file can and will change when new versions of the package that owns it are installed. The result is that the mptcpize modification to that file is unexpectedly lost.

Describe the solution you'd like
systemd provides overrides as a solution to this problem. See https://www.freedesktop.org/software/systemd/man/systemd.unit.html

For example, I suggest that mptcpize enable xyz creates a file at /etc/systemd/system/apache2.service.d/mptcpize.conf with these contents:

[Service]
Environment="LD_PRELOAD=/usr/lib64/mptcpize/libmptcpwrap.so"

Furthermore, using the un-versioned library name (libmptcpwrap.so as opposed to libmptcpwrap.so.0.0.1) will avoid unexpected breakage when the version changes in the future.

[ossama-othman]

@pabeni FYI

[matttbe]

Good idea!
I confirm it would be clearer and it would help in case of software update! :-)