Delete obsolete Solaris compat header file stdlib.h

Delete obsolete Solaris compat header file stdlib.h

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
Approved by:	re (delphij)
Fixes:		9e5787d2284e Merge OpenZFS support in to HEAD
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D38733

(cherry picked from commit 7f0ebf0876c82a098bd5815ea6bf3a8efa6dd6f3)
(cherry picked from commit 95f418fe2a0fb96080afbd2afc160929a32d12fb)