PATH_LIB will search directories in the IDL !path variable in the
order of the list to find a given IDL program. The default is to
only give the first instance that the file is found, but it is possible to
get all locations by using the /multi switch. A wild card (`*')
effectively sets the /multi switch). Same examples are:
IDL> path = path_lib()
IDL> path = path_lib('add*')
IDL> path = path_lib('yodat', /multi)