add some mandoc pages and fix some warnings

- add .Os, it is mandatory.
- don't redeclare .Nm when it's not needed.
- fix some warnings (checked with mandoc -Tlint).
- remove some leftover old stuff.

cron.1

@@ -5,13 +5,12 @@
 .Nm cron
 .Nd clock daemon
 .Sh SYNOPSIS
-.Nm cron
+.Nm
 .Op Fl f Ar file
 .Op Fl n
 .Sh DESCRIPTION
 .Nm
 schedules commands to be run at specified dates and times.
-.Pp
 .Sh OPTIONS
 .Bl -tag -width Ds
 .It Fl f Ar file