remove errno setting from setenv, malloc sets it correctly on oom

fix failure to check malloc result in setenv

fix for setenv bogus var argument handling

thanks to mikachu

per POSIX:

The setenv() function shall fail if:

[EINVAL] The name argument is a null pointer, points to an empty
string, or points to a string containing an '=' character.

initial check-in, version 0.5.0