Adds an actual flu shot.

Changes the in-game name of the current 'flu shot' to 'gamma globulin
shot' and adjusts its description a bit.

Adds rare 'flu shot' that grants actual immunity to the flu. Does
nothing against the common cold.

Also adds a function to both shots to return an empty syringe after use.

Flu shot spawns: shelter 1, hopsital_bed 2, harddrugs, 5