kernel/joy: Remove an unneeded .PATH in the Makefile.
Install pcf(4) manual page on all platforms (just like the module).
kernel: Clean up some module Makefiles.

Adding missing opt_*.h and use_*.h mostly.
strerror(3): Provide an error message for ENOMEDIUM.

Reported-by: Joris Giovannangeli <joris@giovannangeli.fr>
Taken-from:  OpenBSD
libm: Add some parentheses to clarify operator precedence.

This fixes two clang -Wparentheses warnings.

No functional change.

Taken-from: FreeBSD