Cleanup option keyword/environment substitution

Cleanup option keyword/environment substitution

Right now the substitution for options seems quite fragile.  Among the
issues...

- If bc had an error and returned no output, it caused a NULL reference
- Multiple variable substitutions (For example $ncpus * $pagesize)
caused an error as it tried to run bc after the first, with the second
still text
- Memory leak for every keyword substituted
- Multiplication caused shell wildcard expansion (*) of the current
directory when passing the input to bc
- Shell escape sequences would be parsed on the command line when bc is called
- Potential buffer overrun due to unchecked lengths on the input line

So I did a little cleanup to get rid of the issues.  This patch also
moves the environment variable substitution to run before the keyword
substitution, so an environment variable can now indirectly perform a
keyword substitution.

Signed-off-by: Jens Axboe <axboe@kernel.dk>