chfn: rewrite prompt() to use strutils

chfn: rewrite prompt() to use strutils

The left and right white space trimming can be done with strutils.h
[lr]trim_whitespace() functions.

As a minor fix when user input exceeds maxium allowed gecos field length
the remaining characters in stdin are purged so that re-prompting works
correctly.

Additionally the prompt() is made to add message to check_gecos_string(),
so that there are less similar strings for translation project to deal.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>