LIBVIRT_ARG_WITH_ALT is more generic than LIBVIRT_ARG_WITH, which
is tailored at switching features on and off.

Rename the macros according to their intended purpose, and add
some documentation to help developers pick between the two.

Usage of AC_REQUIRE will mess with order how LIBVIRT_CHECK_* macros
are composed into configure.ac.  This ensures that the output of
configure --help is properly ordered and grouped into sections.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

Following patch will update LIBVIRT_CHECK_(LIB|LIB_ALT|PKG) macros in
a way that you will be able to call a another macro as a fallback if
the first one fails.  To allow that, we need to move the
LIBVIRT_ARG_WITH out of those macro to not have two or more same lines
in output of "configure --help".
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

Make it much easier to test a configuration built without readline
support, by reusing our existing library probe machinery.  It gets
a bit tricky with readline, which does not provide a pkg-config
snippet, and which on some platforms requires one of several
terminal libraries as a prerequiste, but the end result should be
the same default behavior but now with the option to disable things.

* m4/virt-readline.m4 (LIBVIRT_CHECK_READLINE): Simplify by using
LIBVIRT_CHECK_LIB.
* tools/virsh.c: Convert USE_READLINE to WITH_READLINE.
Signed-off-by: NEric Blake <eblake@redhat.com>

A future patch will allow disabling readline; doing this in an
isolated file instead of configure.ac will make the task easier.

* configure.ac: Move readline code...
* m4/virt-readline.m4: ...here.
Signed-off-by: NEric Blake <eblake@redhat.com>