top(1): set max username length based on system constant

This changes previous behavior of calculating it at startup based on
the current max username length.

This is done because:

• it is in theory possible for the max length to change at run-time (e.g., a new user is added after top starts running)
• on machines with many users this delays startup significantly

PR: 20799
PR: 89762
Reported by: ob@e-Gitt.NET
Reported by: wkwu@Kavalan.csie.NCTU.edu.tw
Reported on: 2000-08-23 and 2005-11-30