- kernel/register.h, kernel/driver.cc: refactor rewrite_yosys_exe()/get_share_file_n...

 - kernel/register.h, kernel/driver.cc: refactor rewrite_yosys_exe()/get_share_file_name() to portable proc_self_dirname()/proc_share_dirname().
        This refactoring improves robustness and allows OSX support with only 7 new lines of code, and easy extension for other systems.
 - passes/abc/abc.cc, passes/cmds/show.cc, passes/techmap/techmap.cc: use new, refactored semantics.
 - Makefile: no need to add $(PWD) to $(PATH) anymore.