Fix some comments
Add missing support for "weeks" to Seconds2TTL
Create man page in mdoc format. Fix #16.
Use getopts to parse script options

getopts is POSIX compliant and surprisingly portable; even SVR4
supports it.
Simplify "prefixes" join; avoid sed call.
A few tests for script options.
Merge branch 'getopts'
Merge branch 'small_fixes'