mail/mls: improve and unbreak with -fno-common

• liberally sprinkle extern all over the place
• define LICENSE
• take over maintainership of this unmaintained port
• ship examples and html files
• define DOCS and EXAMPLES options
• pacify portlint and portclippy
• add a pkg-message as the author intended
• remove dead MASTER_SITES entry

PR: 259586
Approved by: flo (mentor)
Differential Revision: https://reviews.freebsd.org/D38516