psi_file: psi_load strdup's and stores file path
psi_file.h: Include psi.h header
psi.h: Include string.h header
psi_file: Use own internal strdup method

strdup isn't standard C, so we can't always be sure it has a valid
implementation.
psi: Removing params from main

Just to suppress warnings for the time being.
Makefile: Make obj targets depend on headers
Makefile: Tweak build commands (trivial)