Michał Kępień authored Feb 12, 2019 and Evan Hunt committed Mar 05, 2019

When the "library" part of a "plugin" configuration stanza does not
contain at least one path separator, treat it as a filename and assume
it is a name of a shared object present in the named plugin installation
directory.  Absolute and relative paths can still be used and will be
used verbatim.  Get the full path to a plugin before attempting to
check/register it so that all relevant log messages include the same
plugin path (apart from the one logged when the full path cannot be
determined).

(cherry picked from commit 1a9fc624)