basic: massively reduce the size of arphdr lookup functions

basic: massively reduce the size of arphdr lookup functions

Our biggest object in libsystemd was a table full of zeros, for the arphdr
names. Let's use a switch (which gcc nicely optimizes for us), instead a
table with a gap between 826 and 65534:

$ ls -l build{,2}/src/basic/a6ba3eb@@basic@sta/arphrd-list.c.o
-rw-rw-r--. 1 zbyszek zbyszek 540232 Sep 22 00:29 build/src/basic/a6ba3eb\@\@basic\@sta/arphrd-list.c.o
-rw-rw-r--. 1 zbyszek zbyszek  20512 Sep 25 11:56 build2/src/basic/a6ba3eb\@\@basic\@sta/arphrd-list.c.o

$ ls -l build{,2}/src/shared/libsystemd-shared-243.so
-rwxrwxr-x. 1 zbyszek zbyszek 6774368 Sep 22 00:29 build/src/shared/libsystemd-shared-243.so
-rwxrwxr-x. 1 zbyszek zbyszek 6254808 Sep 25 12:16 build2/src/shared/libsystemd-shared-243.so

No functional change.