We should only execute the filename verbatim if it contains a slash (/)
character somewhere. Otherwise, we need to look through the entries in
the PATH environment variable.
This fixes an issue where you could easily "override" system programs
by placing them in a directory you control, and then waiting for
someone to come there and run e.g "ls" :^)
Test: LibC/exec-should-not-search-current-directory.cpp
