include cleanups: remove unused headers and add feature test macros

slightly cleaner strlen, also seems to compile to better code

testing with gcc 4.6.3 on x86, -Os, the old version does a duplicate
null byte check after the first loop. this is purely the compiler
being stupid, but the old code was also stupid and unintuitive in how
it expressed the check.

initial check-in, version 0.5.0