Fix all compiler warnings on the Debian 10 builds

For 1.0.27, we fixed all compiler warnings on Debian 9 (then Debian's stable release). Debian 10 was released shortly before our 1.0.28 and there are plenty of compiler warnings again. These should be fixed.

Combing through some recent build logs, i.e. those of this pipeline, we have the following warning counts (as counted by [-W occurrences in the logs):

compile	warnings
debian-9-full	0
debian-10-mini	80
debian-10-full	91
alpine-3.10-musl	131
fedora-30-clang	475

Classifying by category for debian-10-mini we get:

category	count	components
cast-function-type	29	sanei_net(1), dll(12x2, !269 (merged)), kvs40xx(1, !229 (merged)), net(3)
format-overflow=	14	plustek(2, !262 (merged)), umax_pp(6x2, !259 (merged))
format-truncation=	3	dll(1x2, !269 (merged)), gt68xx(1, !268 (merged))
implicit-fallthrough	1	nec
restrict	12	umax_pp(6x2, !259 (merged))
stringop-truncation	16	artec(4, !265 (merged)), as6e(1, !266 (merged)), epjitsu(1, !267 (merged)), gt68xx(4, !268 (merged)), hp(1, !279 (merged)), pieusb(3, !282 (closed), !287 (merged)), umax_pp(1, !259 (merged)), scanimage(1, !286 (merged))
sizeof-pointer-memaccess	4	ibm(2, !264 (merged)), ricoh(2, !264 (merged))
tautological-compare	1	hp3900(!280 (merged))

Please note that due to the build process, warnings for dll and umax_pp appear twice.

For the debian-10-full build, the additional warnings are:

category	count	components
deprecated-declarations	2	sanei_usb(1), sane-find-scanner(1)
format-overflow=	1	pixma
format-truncation=	1	pixma
stringop-truncation	4	pixma
tautological-compare	3	hpsj5s(!281 (merged))

Strike out indicates components whose warnings have been fixed.