Helper library with system functions overrides inside a snap

Currently the library overrides the following functions:

• initgroups and setgroups to no-op
• shm_open and shm_unlink to prefix the name with snap.$SNAP_INSTANCE_NAME., as required by snaps

To build and install the library run

$ make
$ make install DESTDIR=/some/path

The snap-preload.so library is intended to be used under LD_PRELOAD, e.g.

$ LD_PRELOAD=/usr/lib/snap-preload.so /usr/bin/myapp ...

Add the following entries to snapcraft.yaml to include the library and automatically provide the preload to all applications in the snap

environment:
  LD_PRELOAD: $SNAP/usr/lib/snap-preload.so

apps:
  snap-preload:
    plugin: make
    source: https://github.com/maas/snap-preload
    source-type: git