Reported by Bruno Haible in
<https://lists.gnu.org/archive/html/bug-gnulib/2024-05/msg00261.html>.
* doc/glibc-functions/getusershell.texi: Mention the musl bug.
* lib/unistd.in.h (getusershell, setusershell, endusershell): Allow the
functions to be declared with the rpl_ prefix.
* m4/getusershell.m4 (gl_FUNC_GETUSERSHELL): Prepare functions to be
replaced on musl systems.
(gl_PREREQ_GETUSERSHELL): New macro.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Initialize
REPLACE_GETUSERSHELL.
* modules/getusershell (Depends-on): Update module conditions to account
for the function being available but replaced by Gnulib.
(configure.ac): Likewise. Invoke gl_PREREQ_GETUSERSHELL.