Build: Separate header install/uninstall

Asterisk headers are no longer installed and uninstalled
automatically when performing a "make install" or a
"make uninstall".  To install/uninstall the headers, use
"make install-headers" and "make uninstall-headers".
The headers also continue to be uninstalled when performing a
"make uninstall-all".

Also corrects an issue where /usr/include/asterisk.h was never
being removed at all.

Change-Id: Ia7399f3a0203a4825fc4a9f43b9034dae9a2b643