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
Reviewed by: markj
Fixes: 9e5787d228 Merge OpenZFS support in to HEAD
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38733