lib: Add eu_tsearch, eu_tfind, eu_tdelete and eu_tdestroy

lib: Add eu_tsearch, eu_tfind, eu_tdelete and eu_tdestroy

Add struct search_tree to hold tree root and lock.  Add new eu_t*
functions for ensuring synchronized tree access.

Replace tsearch, tfind, etc with eu_t* equivalents.

Signed-off-by: Heather S. McIntyre <hsm2@rice.edu>
Signed-off-by: Aaron Merey <amerey@redhat.com>
Signed-off-by: Mark Wielaard <mark@klomp.org>
v2 changes:

This patch replaces v1 03/16 and 14/16.