Improve implementation of install-missing-packages

As suggested by mat@ in D23484, let's use a proper if statement here
instead of short-circuits operators. Also, arguments to echo should be
quoted and ECHO_CMD should be used instead of ECHO (see bsd.commands.mk for
details).

Reported by: mat
Reviewed by: mat
Approved by: portmgr
Differential Revision: https://reviews.freebsd.org/D23535