strbuf: make stripspace() part of strbuf

strbuf: make stripspace() part of strbuf

This function is also used in other builtins than stripspace, so it
makes sense to have it in a more generic place.  Since it operates
on an strbuf and the function is declared in strbuf.h, move it to
strbuf.c and add the corresponding prefix to its name, just like
other API functions in the strbuf_* family.

Also switch all current users of stripspace() to the new function
name and keep a temporary wrapper inline function for any topic
branches still using stripspace().

Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>