mflag: strip quotes in certain forms from flag values

In the form `-flag="val"` or `--flag="val"`, the value is quoted when parsed (while in conventional libraries such as `getopt`, these types of flags are stripped of quotes). This patchset fixed this, and adds some unit tests to make sure that they work.

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)

Closes #6142
/cc @vieux