Fix the execution method of the context

evecve & co. get a NULL-terminated list with the first "argument" being
the command itself (it's a convention). The original code had a few
issues (e.g. it failed to accept commands without arguments). This
pathch fixes those issues by nearly completely rewriting the function.
Merge pull request #544 from neithernut/fix-context-exec

Fix the execution method of the context