Move PL_sh_path to perl.h (except for OS/2), as it's const char[]

Move PL_sh_path to perl.h (except for OS/2), as it's const char[]

On OS/2, keep it in perlvars.h, as it's not const there. makedef.pl doesn't
pay attention to C pre-processor symbols, so it will always see the declaration
in perlvars.h, and add the symbol to the linker file, so no need to mention
sh_path in globvar.sym. Add special case logic in regen/embed.pl to make the
embedvar.h macros for PL_sh_path defined only on OS/2.