Make argument parsing a bit more robust

Currently, when running mfh with a flag like --help the user gets
a confusing error message from expr(1) like the following:

expr: illegal option -- -
expr: usage: expr [-e] expression

revision "--help" should be a number

This is not helpful. Instead, make sure that any arguments specified by the
user are handled properly by expr(1). This results in a much cleaner error
message:

revision "--help" should be a number

Reviewed by: mat
Approved by: portmgr (mat)
Differential Revision: https://reviews.freebsd.org/D25958