plugin: lal_string_triggergen, trigger generator for string search

 - Kipp OK'ed

This reverts commit 9ed1ea2d, reversing
changes made to 3260a519.

this patch makes the mutex a global symbol in the library, which should
allow all plugins loaded by the same process to share a single mutex

- lal_chisquare
- lal_delay
- lal_triggerxmlwriter
- lal_onlinehoftsrc

- don't malloc gap buffers
- add lal_matrixmixer debug category

(this allows me to re-port gstreamer's adder into gstlal with fewer changes
to the upstream version)

Refactored tag registration into a separate library, in preparation for splitting off some elements in the 'gstlal' plugin into smaller plugins

This reverts commit 9db8a0f7.