Remove useless calls to basename().

[ed]

Summary

There are a couple of places in the source three where we call
basename() on constant strings. This is bad, because the prototype
standardized by POSIX allows the implementation to use its argument as a
storage buffer.

This change eliminates some of these unportable calls to basename() in
cases where it was only added for cosmetical reasons, namely to trim
argv[0]. There's nothing wrong with setting argv[0] to the full path.

[ed]

Friendly ping :-)

[jilles]

This may lead to somewhat ugly messages like

-/bin/sh: Syntax error: ")" unexpected

for newgrp -l (where -sh: used to appear) but I think newgrp is not used often enough to warrant making this more complicated.