Closes #665

Writing our own magic bytes detection is prone to errors and an
everlasting catch-up-game. Let's use libmagic to get things right,
this is less code and makes things more reliable.

Building without libmagic is still possible. That will make the code
act like specifying FEH_SKIP_MAGIC=1, effectively passing everything
to imlib2.

This allows feh to load .gif images via libcurl, as some imlib2 versions only
load gif images if the suffix is correct. It's also more convenient when using
--keep-http.

To achieve this, feh needs to use mkstemps. mkstemps is a non-standard
extension that is available on several systems. Compile feh with
"mkstemps=0" to use mkstemp instead.

Closes #630

The --version-sort option requires strverscmp(), but this is a glibc
extension that does not exist on OpenBSD and other systems. To enable
--version-sort on those systems, provide an internal implementation of
strverscmp(). The implementation is from the musl C library and is
MIT-licensed.

The build process remains the same: the verscmp flag should be set to 1
only if strverscmp() is available in libc. If verscmp is 0, then the
internal implementation is used.

Looks prettier