Allow building setmode.c on Linux/macOS

We bootstrap this file to allow compiling FreeBSD on Linux systems since
some boostrap tools use setmode(). Unfortunately, glibc's sys/stat.h
declares a non-static getumask() function (which is unimplemented!) and
that conflicts with the local getumask() function. To work around this
simply use a different name here.

Reviewed By: brooks, emaste
Differential Revision: https://reviews.freebsd.org/D25929