build: use common .in replacement mechanism

build: use common .in replacement mechanism

We had several different styles of .in conversion in our Makefiles:
ALLCAPS, @ALLCAPS@, @lower@, ::lower::
Canonicalize on one form, to make it easier to copy and paste
between .in files.

Also, we were using some non-portable sed constructs: \@ is an
undefined escape sequence (it happens to be @ itself in GNU sed,
but POSIX allows it to mean something else), as well as risky
behavior (failure to consistently quote things means a space
in $(sysconfdir) could throw things off; also, Autoconf recommends
using | rather than , or ! in the s||| operator, because | has to
be quoted in shell and is therefore less likely to appear in file
names than , or !).

Fix all of these uses to follow the same syntax.

* daemon/libvirtd.8.in: Switch to @var@.
* tools/virt-xml-validate.in: Likewise.
* tools/virt-pki-validate.in: Likewise.
* src/locking/virtlockd.init.in: Likewise.
* daemon/Makefile.am: Prefer | over ! in sed.
(libvirtd.8): Prefer consistent substitution.
(libvirtd.init, libvirtd.service): Avoid non-portable sed.
* tools/Makefile.am (libvirt-guests.sh, libvirt-guests.init)
(libvirt-guests.service): Likewise.
(virt-xml-validate, virt-pki-validate, virt-sanlock-cleanup):
Prefer consistent capitalization.
* src/Makefile.am (virtlockd.init, virtlockd.service)
(virtlockd.socket): Prefer consistent substitution.