Replace strlen() with strnlen()
Check argc against MAXARGS in loop condition
Define MAXARGS in terms of MAXLINE
Fix argv bug

execve() requires argv to be null-terminated. This was not being done
previously. Args from older, longer commands were being incorrectly
passed to execve().

Example:

    > echo 1 2 3
    1 2 3
    > echo 1
    1  3
Add comment about the mysterious environ variable
Merge pull request #1 from jayschwa/code-review

Code review