Happens e.g. when using -O without enabling thumbnail/index mode

Closes #335

The check if buffer == NULL is always false, since buffer is an
autoamtic variable allocated when entering the function.  What we
instead want to do is to check if the string is empty after the call to
exif_get_info(), since that means we could not read any exif
information.
When the code once more is enabled, I discovered that we need to copy
the information string into info_buf as well as into  buffer, since it
is the former that is used to print the exif information on top of the
picture.  Without this change, imlib warns about trying to write NULL
strings.

Closes #326

When feh loses its controlling terminal at runtime, e.g. due to backgrounding /
disowning, it will no longer issue a warning on each terminal keystroke.

This allows finer control of which files are compiled and additionally
will silence some -Wpedantic warnings when "empty" files are compiled.

As there's no "default" window, it doesn't make much sense here...
Plus, it's buggy as hell in this mode

Also, restore the previous terminal mode on exit

Turns out that it is undefined behaviour to pass a value to isctype functions
which does not fit inside a char. Closes #312

(cf #309)

Closes #303

If malloc cannot allocate enough memory, it could return NULL. This is
not necessarily true for default Linux settings, but can be provoked
there as well by adjusting proc entries. Other systems like the *BSD
ones definitely do this.

The function _emalloc exists for exactly this purpose, so use it instead
of calling malloc directly.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

If feh_unique_filename encounters a file that already exists, the memory
for the temporary filename is not released. As this happens in /tmp at
some code places, an attacker could use this to spray the memory of feh,
or simply triggering an out of memory condition.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

If ereadfile encounters an empty file or the file could not be read, an
out ouf boundary read (and possible write) occurs. Always check the
return value of fread to be > 0 before processing the result buffer.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

The strncpy function does not guarantee to end the resulting character
sequence with a terminating nul character if not enough space is
available. This could be triggered by supplying a sufficiently long
output_file option.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>