b654714f has reworked the way trailing CR/LF characters are removed from
strings.  This commit introduces a new routine in common/string.c and
refactors the code so as the logic is in a single place, mostly.

Author: Michael Paquier
Reviewed-by: Bruce Momjian
Discussion: https://postgr.es/m/20190801031820.GF29334@paquier.xyz