make C99-compatible

Since the project is supposed to be C, it makes sense to make it compile and
link as such. I've eliminated C++-specific headers, fixed the source file
extensions, removed some evil macro defintions ('#define NULL 0'?), etc.

Next step is to make this C89 compatible so that tools like Microsoft Visual
Studio will compile it as C code without complaint.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
cosmetic: make spacing and indentation consistent

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
cosmetic: change C++ comments to C comments

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
headers: add multiple-inclusion guards

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
headers: include appropriate headers which declare structure types

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
sources: move variable declarations to head of functions for C89-compliance

ISO C90 (which Microsoft's C compiler still uses) forbids mixed declarations
and code.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
jumppointsearch.c: make sure main() returns an exit code

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
path.c: eliminate unused variable 'i'

Signed-off-by: Steven Noonan <steven@uplinklabs.net>