ps(1): Pet mandoc and igor

• Use Xr to reference other manual pages.
• Reference execve(2) instead of exec(2) as exec(2) does not exist.
• Remove the deprecated "Tn" macro.
• Improve the formatting of the etime description.

Reviewed by: bcr
Approved by: krion (mentor, implicit), mat (mentor, implicit)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D17780