lib/mu-str.c: squash white space ctrl chars to spaces

When processing multiple lines for a subject line separated by TAB
characters we don't want to eliminate the control character totally but
replace it with a simple space. I've left the control handling as before
for non-white space characters.

Signed-off-by: Alex Bennée <alex@bennee.com>
Merge pull request #422 from stsquad/remove-ctrl-squash-space

* lib/mu-str.c: squash white space ctrl chars to spaces (#390)