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)
* mu4e/org-mu4e.el (org-mu4e-store-link): Add capture template keywords

For other email links (e.g., wl, mh, gnus), a number of keywords are
defined in org capture templates.  This makes these keywords available
for use in captures from mu4e.

http://orgmode.org/manual/Template-expansion.html

Inspiration from Roelof Burger
https://groups.google.com/d/msg/mu-discuss/4sffZofVKAY/aVTdVYoVyiUJ