Cleanup: return gasmix by value

Currently, get_gasmix_from_event() and get_gasmix() return pointers
to either static or to (possibly changing) dive data. This seems like
a dangerous practice and the returned data should be used immediately.

Instead, return the gasmix by value. This is in preparation of
const-ifying input parameters of a number of core functions, which
will ultimately let the merge() function take const-arguments in
preparation of undo of dive-merging.

On common 64-bit systems gasmix (two "int"s) is the size of a pointer
and can be returned in a register.

On 32-bit systems a pointer to the struct to be filled out will be
passed.

Since get_gasmix() now returns a value, the first invocation is
tested by a NULL-initialized "struct event *". Document this in
a comment.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

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