Use the correct idiom for default values, and ensure that the script
works correctly if the user overrides them.

PR:		193255
Submitted by:	hrs@
MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271257 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f