This brings mk's behavior when using rc in line with Plan 9's. The existing code is for Unix environment data structures but also was assuming Unix shell semantics where empty and missing variables are mostly equivalent. The Plan 9 code (/sys/src/cmd/mk/plan9.c in the distribution) explicitly removes /env/name (creating an empty list) when the value is missing or an empty string. Fixes #255.

Remote whitespace at the ends of lines. Remove blank lines from the ends of files. Change modes on source files so that they are not executable. Signed-off-by: Dan Cross <cross@gajendra.net>

syntax will change.