src/main.c: switch spaces to tabs to clobber GCC warning

The if statements are needed because we cannot pass a NULL pointer to
XFree.

src/feh_png.c: close file upon error

Add description of option "--no-recursive" to man page.

Purpose of "--no-recursive" option is disabling effect of "--recursive" option that enabled by user's theme.

Turns out giflib 5.1.2 is the culprit and it isn't Deban-specific after all.