tidy up some extern declarations

Convert to use C89 prototypes.
Remove use of __P

branches: 1.3.6;
Remove 3rd and 4th clauses in christos' license. OK christos.

If port provides __weak_alias(), provide an Standard C and POSIX pure
identifier namespace by renaming non standard functions and variables
such that they have a leading underscore. The library will use those
names internally. Weak aliases are used to provide the original names
to the API.

This is only the first part of this change. It is most of the functions
which are implemented in C for all NetBSD ports. Subsequent changes are
to add the same support to the remaining C files, to assembly files, and
to the automagically generated assembly source used for system calls.
When all of the above is done, ports with weak alias support should add
a definition for __weak_alias to <sys/cdefs.h>.

Fix RCSID's
printf -> warnx
Fix gcc warnings
Put locally declared functions in local.h