* ltmain.in (progpath): Work around backward compatibility issue

* ltmain.in (progpath): Work around backward compatibility issue
on IRIX 6.5. On IRIX 6.4+, sh is ksh but when the shell is invoked
as "sh" and the current value of the _XPG environment variable is
not equal to 1 (one), the special positional parameter $0, within
a function call, is the name of the function. So, rather than
using "$0" in functions, we set $progpath in the body and use that
everywhere instead (incase of code refactoring later).
(EXIT_SUCCESS, EXIT_FAILURE, EXIT_MISMATCH): For clarity.