checking static vs extern symbols

[Paul Eggert <eggert@cs.ucla.edu>]

No further comment, and it appears that there wasn't consensus
to mark DEFUNs as to whether they were static, so I removed that
part of the change.  Also, I removed the INFUN macro too, as
Dan Nicolaescu suggested.  What's was left, was changing several
C functions and variables to be 'static', plus removing some
code that was thereby exposed as being unused.  I merged this
to the trunk as bzr 103913.

I have an approximately 100-line shell script that reports
C variables and functions that are exported but shouldn't be.
It finds every extern symbol that is not used by any
other module, and isn't one of the exceptional cases (the
biggest exception being DEFUNs).  I plan to make this shell
script run as part of a new 'make maintainer-check' action.

I could call this shell script 'src/extern-check', or could
give it other names (in other directories) if people would prefer,
but the basic idea is that "make maintainer-check" is intended
to be used by maintainers who are running on platforms
that have the proper tools available, and that "make maintainer-check"
reports glitches such as unwanted exports.  (I have some
other maintainer checks in mind, but one step at a time.)