net-mgmt/nagios-check_smartmon: Fix ValueError with some drive values

After upgrading to version 20100318_4, users reported ValueError for
some disks [1]

Also fix Python 3.x compatibility [2]

While I'm here:

• Pet portlint, NO_* in USE{S} section
• Make concurrent-safe, installs executables in libexec

PR: 236873
Reported by: Thomas Eckhardt <freebsd eckieck de> [1]
Submitted by: Thomas Eckhardt <freebsd eckieck de> [1]
Submitted by: Krzysztof <ports bsdserwis com> (maintainer) [2]
Approved by: Krzysztof <ports bsdserwis com> (maintainer)
MFH: 2019Q2 (bugfixes)