Better check for existing spgwu processes

[nkakouros]

When running the spgwu there is a check that no other instances are running:

https://github.com/OPENAIRINTERFACE/openair-spgwu-tiny/blob/master/src/oai_spgwu/main.cpp#L84

This creates a lot of usability problems. If you run any other process that has spgwu in the name or as part of its argument list, spgwu will fail to start.

A more robust way could be to check the /var/run/ folder for a pid.