[SV 63045] Reload each intact unloaded shared object

[SV 63045] Reload each intact unloaded shared object

If makefile rules do not update an unloaded shared object, load it
again.  Avoid double loading of the same object if the setup function
returns -1.

* src/filedef.h (struct file): Add "unloaded" flag.
* src/makeint.h (load_file): Take struct file *.
(unload_file): Return int.
* src/main.c (main): Reload unloaded shared objects if they weren't
updated.
* src/commands.c (execute_file_commands): Set "unloaded" and reset
"loaded" when a shared object is unloaded.
* src/read.c (eval): Set "loaded" and reset "unloaded" when a shared
object is loaded.  Add successfully loaded files to the db.
* src/load.c (load_file): Check "loaded" to avoid double loading the
same object.  Fix a memory leak of string loaded.  Return -1, rather
than 1, if the object is already loaded. This fixes double loading of
the same object when the setup routine returns -1.
(load_object): Add a log message.
(unload_file): Return an error on dlclose failure.  Log a message.
* tests/scripts/features/loadapi: Add new tests.