This drops function getexecname() redirection.
Historically getexecname() is a compatibility definition. Since openzfs has its own implementation of function getexecname() in libspl and has been merged into base, the compat header file stdlib.h is no longer needed and should not be used.
Also without this fix libspl will end up an incompatible version of getprogname() with libc. In particular, if zfs is enabled, programs such as pgrep in /rescue can be wrongly statically linked with libspl and will not function properly.
PR: 269738
MFC after: 1 week
Fixes: 9e5787d2284e Merge OpenZFS support in to HEAD