Adding getprogname and setprogname for all BSDs and Solaris.

Adding program_invocation_short_name for Linux (Musl and glibc).

Adding __progname for Android. This is a little different, but
is a safer alternative to using argv[0], which may not exist, and
includes Android's default application name (currently '<unknown>').

Adding these functions and externs means it is possible for all
but Windows applications to safely discover their
name, rather than rely on argv[0] parsing, /proc/self/exe, etc.