pkg: mflag: flag: make mflag strip quotes in -flag="var" forms

This patch improves the mflag package to ensure that things arguments
to mflag such as `-flag="var"` or `-flag='var'` have the quotes
stripped from the value (to mirror the getopt functionality for similar
flags).

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
pkg: mflag: flag: added tests for quote-stripped flags

This patch adds some tests to ensure that quoted flags are properly
handled by the mflag package.

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)