gdb/dwarf: replace some "compile unit" terminology with "unit"

gdb/dwarf: replace some "compile unit" terminology with "unit"

In DWARF 5 (and even previous versions, with type units), compile units
are just one type of units.  In many places, we still use "compile
units" when in reality it would be better to talk about "units" (unless
we specifically want to talk about compile units).

Rename comp-unit-head.{c.h} to unit-head.{c,h}, and do a big pass of
renames in it to remove the specific mentions of compile units, where in
fact we want to talk about units in general.

Change-Id: Ia06c90ccb25756c366f269a12620f2f7c8378adb
Approved-By: Tom Tromey <tom@tromey.com>