lib: rename Prog to shadow_progname, with only one definition

lib: rename Prog to shadow_progname, with only one definition

The build was failing with duplicate symbol errors with -fno-common.
This is the default in GCC 10 and later, and explicitly enabled in some
distributions to catch problems like this. There were two causes:

- Prog and shadow_logfd were defined in a header file that was included
  in multiple other files. Fix this by defining them once in
  shadowlog.c, and having extern declarations in the header.

- Most of the tools (except id/nologin) also define a Prog variable,
  which is not intended to alias the one in the library. Fix
  this by renaming Prog in the library to shadow_progname, which also
  matches the new accessor functions for it.