Create simple non-thread-safe "gasname()" function

Every single user of the get_gas_name() function wanted to just use the
result to create a message or printout, and that made the interface a bit
cumbersome.

This adds a "gasname()" wrapper that returns a static buffer with the
result in it, which is neither pretty nor thread-safe, but is a much
simpler interface to work with.

Our dive planning isn't multi-threaded, so nothing should care.

[ This also converts a few open-coded gasname debug printouts to use the
  helper function, resulting in prettier printouts and less code ]

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>