Fixes alien embryos from runtiming when checking limbs

affected_mob was null when it checked for limbs.
Fixes multiple embryos in single host

Added checks so you can’t have multiple embryos in a single host.
Checks are included in both facehugger.dm and alien_embryo.dm, because
there might be ways to implant embryos without facehuggers in the
future, but we also want to avoid creating and instantly deleting
objects.
Also changes chest augmentation surgery to remove any embryos. Before
you could remove the whole chest without finding the embryo inside.
Changed Del to del

Proc names are case sensitive
Changed bitflag 'or' to boolean

| != ||
Deleted checks from alien_embryo

No double checks.
Merge pull request #1936 from Perakp/faceHuggerRuntime

Fixes alien embryos from runtiming when checking limbs